Exactly - alot of sites start taking donations or subscription payments via paypal. When you reach the point that it's too big a pain to manually add everyone to a special user group - then this is for you.
The site this was initially develope for (one of mine) was getting dozens of signups a day. Also if you're charging for access of something, then instant access is a definate plus, vs. waiting for the admin to add them into the user group to gain access.