1. I don't have to write code or maintain a server to process payments. I realize that I'm an outlier for not wanting an API, but my business web page is completely static, and it's convenient for me to keep it that way.
2. They are unified with USPS for shipping, which has the best rates and service for packages that weigh just a few ounces. I can click on a button in PayPal and it spits out a USPS shipping label for me.
I greatly prefer not having to handle any of my customers data. The only info I ever see is their shipping address and whatever e-mail address they signed up to PayPal with.