you probably sign a license agreement: there's no "As long as you understand"

Yes, to gain access to the GitHub repo you need to sign up for an Unreal Engine license which involves accepting this eula:

https://www.unrealengine.com/eula

Correct. I think people do this when they need new features/fixes now. In some cases you gain the future benefit of the maintainers being more open to your issues/feature requests.