Some sellers take Paypal and not debit/credit. There are any number of reasons to use Paypal.
But why is this much different from already using Google Checkout? Paying on the market is so easy and it stores my info securely.
Once the market can use Paypal, I will actually start paying for apps just due to the simplicity of it. I'm not about to punch in my CC number each time I want to buy something for a couple bucks.