Successful contributions to open source projects are a kind of currency. GitHub in particular encourages this in a number of ways: by showing avatars of contributors on repository pages, by showing your contributions to your followers via the activity feed and by signalling contributions per day on the activity graph of your profile. Potential hiring managers often take note of this. Recruiters often find and screen candidates this way. If you are a software developer (either existing or aspiring) looking for work, tuning these signals can often work to your advantage.