Pytest: Why not put __init__.py in test dir?

I'm reading through pytest docs and I've come across this part which I don't understand:

avoid “__init__.py” files in your test directories. This way your tests can run easily against an installed version of mypkg, independently from the installed package if it contains the tests or not.

Perhaps a good explanation would say what would happen differently if I had __init__.py in there vs if I didn't. Thank you :)