If you contribute patches to the Drupal issue queues, you often spend a lot of time and hard work getting that patch through the review process. Even though we do it for the good of the community, once it is all said and done, it's nice to get a little acknowledgement. While there is a page for module maintainers on how to properly attribute contributions to users, not all maintainers know about that, and it can often mean they have to spend a little extra time figuring out what your Drupal.org git email address is.
Learning to develop professionally involves a lot more than just writing code. Major required skills include keeping code stable long-term, sharing tasks within a team, and building understandable interfaces so your code can be connected to and run from other programs. Learning these skills is going to involve a lot of mistakes, but fortunately there are tools out there to help you get you ahead of the curve before joining your first team.