Some people just want to contribute to a common cause. Some people just want to spend five minutes of their time to fix that one annoying papercut which been bothering them for ages, others just enjoy working together with friendly people to achieve something tangible they can be proud of.
In my opinion one of the biggest benefits of Github to the open-source community is that contributing now has extremely low barriers. It is absolutely trivial to put an "edit this page" link in your documentation which lets a complete stranger fix a typo and open a pull request within seconds - and seeing your fix go live within a few hours is absolutely magical. That kind of trivial contribution is a gateway drug to becoming a valued community member, and it is absolutely essential to maintaining a healthy ecosystem around open-source projects.
In my opinion one of the biggest benefits of Github to the open-source community is that contributing now has extremely low barriers. It is absolutely trivial to put an "edit this page" link in your documentation which lets a complete stranger fix a typo and open a pull request within seconds - and seeing your fix go live within a few hours is absolutely magical. That kind of trivial contribution is a gateway drug to becoming a valued community member, and it is absolutely essential to maintaining a healthy ecosystem around open-source projects.