Two reasons. First, they want to push people to open source small personal projects and contribute to large OSS projects. Doing anything with an open project,

Two reasons. First, they want to push people to open source small personal projects and contribute to large OSS projects. Doing anything with an open project, even just filing bug reports, counts towards your streak. Second, it's more of a novelty thing since your contrib count is very easy to hack. Just generate a local repo with commits on the days you want and upload it.