There seem to be tangibles and intangibles, extrinsic rewards and intrinsic ones as well, it all depends on who you are and what symbolic things mean to you, not to get to Zen or existential...
I seem to be a "joiner", I like to join worthy groups and help out, that is a reward I feel inside, the access you get to the "Legends of the Road" and their wealth of experience is also worthwhile. Formalizing your role in a group of like-minded gear-heads is just one way to take it to the next level... I take pictures and write articles, the ISSCA magazine is a great forum for that activity. I like the newsletter, learning what others are doing across the country.
The Internet and the Web have made it much easier for folks to reach out to others, so the local nature of car clubs remains but has changed, but now we see folks swap tech ideas on the F-body BBS and others... the drag race and Englishtown NJ included members of many clubs in the Northeast region, not all of which were ISSCA members, but that is OK.
I think ISSCA does a good job of catering to a wide variety of interest levels, you can pay your dues, sit back and read the newsletter, or you can attend events for the members (they have been doing a great job of making non-members feel included and welcome, strangers are just friends ya ain't met yet!)... if you are further motivated you can volunteer to help at an event, judge a car show, organize one yourself, etc... or aspire to a leadership role within the national or local clubs.
This national Impala group seems to be more open, inclusive and less judgemental than the old one (thank God!), so in my eyes, they are worthy of support merely because of what they are not.