The bi-monthly e-mails that Enterprise sends out nearly always include a free upgrade coupon as it is, so why would I need to sign up for "Enterprise Plus"?

I guess what I find confusing is that the counter agent suggested to me that "Enterprise Plus" rewarded renters based on how often they rent from Enterprise (similar to the way you accumulate frequent flier miles, etc). However, I see no reference to this in the FAQ's.