If PayPal knows you have used PayPal this is what your checkout screen looks like:

PayPal knows you have an account or have used their service before.  They don’t even give you the option to pay with a credit card.  They will make way more money if you pay with PayPal.

This is what PayPal looks like if you don’t have a PayPal cookie on your computer:

Notice how they offer you to enter in a credit card this time.  They know you aren’t familiar with PayPal   and logging into PayPal is a major drag.  This way they can grab the sale as soon as possible.  (Although the next step after entering your billing information they will ask you to open a PayPal account).