Open Source projects rule the world with their [usually] good code quality, but more importantly because they are available for free. This also means that the usage-to-contribution ratio is very low,
In other words, a few hundred contributors work on maintaining/improving those open-source projects compared to thousands or millions of users.
Hacktoberfest is one such event from DigitalOcean that encourages you to contribute back to your favorite projects. In exchange for your contributions, you either get a gift from DigitalOcean, or you can choose to plant a tree instead.