You see, if you just use Paypal buttons you are not providing any information on the checkout page other than what Paypal provides. Most buyers would be ok so long as you give them good information on the sales page, however with OptimizeMember, the process can get quite confusing for your customer going that route.
This is due to the fact that they would have to find their way back to your site (they will be automatically redirected if everything goes perfect) and then register after they land on an extremely ugly page with an uglier button so they can fill out a form to register.
Half a dozen emails and several times visiting links to your site to get their username and password, they might finally be able to access what they purchased (or gave up and asked for a refund).
Pro forms take care of registration and payment at one time.
If you are using paypal you don't even need a Pro account to use Pro forms (if you have questions about this I'm happy to explain later).