First, there's the convenience factor.  You have to have a PA account to use a PA plugin, so if you're buying from a third party, you have to create two different accounts.  When you buy a PA plugin from Sweetwater, what you're actually buying is a voucher code that will bring your price for that plugin down to zero.  So that means you'll have to complete the purchase process in both accounts. Also, buying from a third party will make things more complicated if you need to do a return or exchange. 

Our plugins are usually very stable, but if you do have problems and want a refund, you'll have to go through Sweetwater. 

The biggest reason to buy from us is that we periodically have special "Customer Loyalty" deals, where we give special discounts to our bigger-spending customers.  But if you buy all your plugins from third parties, we have no way of knowing how much you spent, so we won't be able to include you in these deals.