If you don't ever use it, then I suggest that you uninstall it. It's just taking up memory and storage space and you're not using it.
Some apps do run in the background. I have no idea why PayPal has set their app to do this, but they apparently have. As, I don't use it, or even have it installed, I don't know why it is running in the background or what it is doing there.
When I get some time (probably a couple days) I'll install it and see if I can figure out what it is doing.
Kratos