I've belonged to one for at least 5 years now, and my reasons are both political and personal--and spiritual.
Political in that I don't want giant agribusinesses to ever gain control over all food production. They've got an iron grip around most of it, from controlling seed production to ownership of way too much land. Instead, I'm pleased to know that I'm helping keep in business Green Thumb farm, a family-owned enterprise on Long Island's gorgeous North Fork.
From a personal perspective, this means I don't have to worry about being poisoned by a vegetable. The folks at Green Thumb follow strict organic methods, and plant heirloom varieties of great-tasting fruits and vegetables. Agribusinesses grow varieties that lend themselves to mechanical picking and repeated handling. That's why, for example, you need a super-sharp knife to cut into a supermarket tomato grown for its tough skin, not taste.
Green Thumb picks my veggies the day before they deliver them to a local church, where we pick them up. As a result, even if I can't use them immediately, they stay fresh in my refrigerator for much longer than supermarket produce, which often travels thousands of miles before landing at the local market. Distribution of fruit and vegetables has become really insane, with tomatoes picked in Florida getting shipped to Mexico for sorting and boxing before being shipped back to the U.S.
Sometimes Green Thumb delivers veggies I've never seen before, like salsify, which looked to me like just a bunch of twigs. (Actually, it's a root that can be cooked and mashed.) Once I even had to go on line to try to identify some of the veggies I received. They had been labeled at the pick-up point, but they looked alike to me! But I appreciate having to stretch my culinary skills as I try to move away from meals centered around meat.
All this costs me about $20 a week for things like strawberries (in season only!) that are really sweet and free of pesticides; sugar snap peas; beets, various kinds of lettuce, arugula, etc.
The spiritual part has to do with being in rhythm with the earth and the seasons, knowing as the summer progresses that the lettuce has gone to seed, and won't be good again until the weather cools, but the tomatoes--ah, the tomatoes, will be wonderful--while in fall I'll see amazing varieties of cauliflower and other wonders. For me, this is all about the mystery of life and living on planet Earth.