What is this code for?

That tells PayPal that the payment came from a button created with this plugin.

Maybe I should have phrased my question better, WHY is this being used? Why does my site need to tell PayPal that the button was created with this plugin?

Your site doesn’t need to. My plugin does. If you want me to keep building free plugins like this then PayPal needs to see that the work I’m doing is benefiting them.

Definitely understandable!

??