It is a very fast way to do payments. With Dynadot, I have to enter all of my credit card information, and then also enter my CC number (tedious and error prone) every time I want to make a transaction. I'd much rather just click a Google Checkout button instead.
Namecheap has this feature, and it's a real time saver, and takes away the annoyance of filling out more forms.
Using Amazon simple payments would be even better.