The local church's need for classes and the denomination is a bit like a believing family's need for a congregation. A believing family should not just be content with worshiping and studying together, they need a church to attend, a gathering of other believers where they can worship, study, learn and hold each other accountable together. The same is true for churches. It might be tempting to think that a congregation only needs those members within it, but just as an individual family needs to meet with other believers, so churches need to be in fellowship and communion with other churches, learning, growing, worshiping and holding each other accountable. We need each other and we need other churches who hold to the same doctrine and teaching so that we can figure out what it means to live out God's mission in our own contexts.
Classis is that extended family for our churches. I don't think this means that everyone in the pew needs to be directly involved in classis but the local church as a whole needs other churches. Are our classes being the type of extended family we all need? What could we be doing better?