I'll go first. I'm the opposite of a social butterfly, and not a joiner at all. My business comes from word-of-mouth referrals, and most outbound marketing campaigns are a waste of effort. But I joined the Chamber as soon as I moved here.
My membership does not generate new business, or even inquiries. Instead, I get to meet smarter people, and learn from other local businesses. In turn that helps me understand my customers' businesses, and how best to support them.
The benefit for my business comes from local wisdom.