2. Magics that are greater than the sum of the parts
I remember reading somewhere (oh, how I wish I could remember what book that was...) that one person creates 1x magical energy. Two people create 2x magical energy. Three people create 4x magical energy. Four people create 16x magical energy. And so on goes an exponential growth of magical energy as more people are added to the coven, until a critical mass is reached at which point the law of diminishing returns sets in; Too many people creates too many possibilities for stray thoughtforms and energies, and after a certain point, adding more becomes counterproductive. Though magical energy is hard to quantify in these kinds of terms, I have observed that with more people working in tandem, focusing their energies toward a single goal, the levels and vibrations of the magical energy can reach astounding heights.
Many covens, including my own, work skyclad, as a means to further remove psychic barriers between the coveners. This, coupled with the removal of barriers through the establishment of love and trust, means that the energies of the coveners can merge in a coherent, harmonious manner, amplifying one another to heights that would not be able to be achieved on one's own. Within a coven, the magic worked truly is greater than the sum of its parts.
3. A setting that meets the individual's needs
Let's say that your grandma is sick in the hospital. You go to your local Pagan church and tell them this, and they give you the standard, "We'll light a candle or send some energy at the next event." That'd be very helpful, except for the fact that eight other people have likewise come to the church with requests to focus on their own needs. Who gets priority? Is it first-come-first-served? Or do they just do a wide-spectrum scatter of energy and hope enough gets raised by the motley of people who may or may not care about your situation that it has some actual impact on Grandma's well-being?
This is rarely a concern in a coven. If I have something planned for my coven to work on at any particular esbat, and a covener has a working he or she needs to do immediately, I can shift things around. I can make sure that everyone's needs are addressed in a timely fashion, and work with the coveners to ensure the best possible spellwork to achieve their goals.
4. A place to accomplish your own goals
We witches are a very diverse lot. Some of us really love to spend our time learning about herbs, potions, and balms. Others are fascinated by energy work and energy healing. Still others really like to focus on divination and scrying, while others still love going on flights through the realms of spirit, and I've barely even touched on all of the many fascinating facets to the Craft. Within a small group, we can take time to ensure that each person has a space in which they can pursue their own magical goals. Often, we will rotate who is doing what, and what our focus is on that particular esbat.
So, within a large pan-Pagan format, when and where do you find a place to explore your interest with others? Do you have to wait until the few dozen or so other people get a chance to do their thing? Or until the leaders turn their eyes your way? How exactly are you going to get your personal interests on the table as a topic of exploration? Larger groups mean it's harder to cater to everyone. No such worry in a coven; there's time enough for everyone to explore what they desire.
We have a very strict rule in our coven, and it's one I take with the utmost seriousness: what happens in the circle stays in the circle. At the last pan-Pagan gathering I attended, someone set up a video camera, so obviously, that rule wasn't in effect there. Even if there are no video cameras, there's a circle full of strangers who may or may not repeat what you said to others. I wonder how someone who needs to use that circle to deal with a depression issue, a health problem, a relationship difficulty, an addiction, or a work-related trouble might feel, knowing that their very personal, very difficult words are potentially going to be spread far and wide.
Furthermore, many of us require privacy. Some have employers who would not be too pleased to see their employees engaging in rituals that may well terrify their sensibilities. Some have vindictive exes who would gladly and gleefully use such a video, indiscriminately placed on Youtube, to wreak havoc in the person's life. Some of us take that lesson of the north, "to be silent," to heart, and prefer to stay private about our activities. Regardless of reason, nobody should be forced to be any more out of the broom closet than they are comfortable. If your religion puts your personal, professional, financial, or familial well-being at risk, then that religion needs to be reassessed; A spiritual practice should be bettering your life, not the other way around. A coven can assure your privacy at whatever level you are comfortable with.
6. Coherency of paradigms and practices
Pagans can't even agree on what Paganism is. You need to believe nature is sacred... or not. You need to worship multiple gods... or not. You need to be environmentally conscious... or not. You need to practice magic... or not. You need to work ritual in this particular format... or not. You need to be around this place on the political spectrum... or not. You need to do these certain practices... or not. You need to celebrate the Wheel of the Year... or not. You need to have this understanding of the mechanics of the universe... or not. There's absolutely no coherency. Looking around the Pagan world, I see many, many people with whom I have nothing in common. How can I share a meaningful spiritual experience with people who don't even agree on what a meaningful spiritual experience consists of?
A coven solves many of these issues. Simply put, within a coven, only those who fit the coven's agreed-upon practices and share a sufficiently similar spiritual paradigm are extended welcome into that group. In a coven, nobody is asked to put aside their personal paradigms or practices to mesh with the group - the group's practices mesh with the personal practices and paradigms of the coveners, else they wouldn't be there in the first place. By the use of one set of rituals for the group, the group's egregore becomes strengthened, and the energies of the group become more harmoniously attuned. Initiation rituals further solidify this bond; everyone who is in the coven has undergone the same ritual, and have that shared experience as their starting point within the coven.
And one bonus reason: wine
Pan-Pagan gatherings almost never serve wine. Covens typically do. Mine certainly does. Cakes and grape juice just aren't the same.
Coven work is not for everyone, but for some of us, nothing can replace it
Humans are not a one-size-fits-all kind of species. Each one of us has different needs and seeks our spiritual practices with different goals. I have attended a good number of open pan-Pagan gatherings, but rarely have I ever walked away feeling as incredible as I do after a circle with my coven. I need the intimacy and depth that coven work offers. For me and many other witches, nothing can replace the feeling of working in a coherent, cohesive, close group. While many might flock to the exoteric gatherings, there will always be those of us who need to tread the more esoteric ways.