I will admit I have learned a lot from this site and bought and sold a few things, that said the honest reason I joined is because of the discount. A LFS pointed the site out to me and told me about the discount, I joined and made back the $25+ the first purchase and I have been saving money since. Wife and I delayed renewing this year because we figured we wouldn't save $25 this year, weighed it out and thought we would come close enough to make it worth while.

Short answer to save money.
Clif