Essentially once you join and spend over $20k a year, you start to receive discount benefits. There are sometimes promotional discounts, usually for business class bookings. Right now they are running 30% off certain domestic business class bookings.
I am a member and find it pretty useless. Their website portal is a joke, and makes bookings very difficult. Even though I struggled through it, I get monthly statements showing my spend, and they show all zeroes. So Accelerate doesn't even recognize what I have been doing.