I am relatively new to GORC, but would echo many of the sentiments posted in this thread.
Riding a mountain bike is definitely a great way to stay in shape, but it's fun and doesn't really seem like exercise in the way that say, going to the gym is exercise. To get the chance to ride on a trail that you built definitely increases that enjoyment level.
I would say that building trail allows you to give back to the biking community and in general the outdoor community in St. Louis. There is also the added social benefit of meeting other trail users at workdays and hanging out afterwards. Furthermore, the more miles of trails in St. Louis will lead to either more trail users or less congested trails and I would be happy with either outcome.
It is a pretty awesome feeling to hear people (who might not even know you helped with the trail) talk about how awesome the new Zombie trail is in Eureka and how much fun they have riding it. It's nice to know you directly added to the enjoyment.
I would add one more comment for a personal benefit that I have not seen yet and wasn't necessarily seeking when I joined GORC, but it totally increases your general appreciation of trails that you might have not even helped with. Whether it's riding out in Colorado, hiking out in Wyoming, or just riding the Ozark Trail in Missouri, all of those trails were built by the effort of trail builders. Joining GORC has increased my appreciation of all trails, what went into them, and how fortunate we are to have them.