Maybe you are a young person, and don't have access to online payment methods. $84/year or more could be really significant for a 15 year old, or someone in a country that Github can't legally transact with.

Or maybe you need to roll your own hosting solution. Maybe for security, maybe you're integrating with APIs like a corp SSO that are not on the public web, etc.

We do a few wacky things in our business (test infrastructure SaaS) and sometimes we need to do stuff at scale where all available commercial solutions assume a single user. Open source solutions are a godsend there. We don't have that need for code repos (we use Github) but I can totally see why others would need it.