This feels a bit like asking why I would send my coworkers a Slack message instead of an email. For some things, send and wait for them to look at it is fine, but if I want to have a conversation, it's a lot less work to use something that's real-time. If we're both working on nice modular features, and they just need to use the code I wrote as-is (or only make some small targeted changes that they can do without my help), git branches are useful. Sometimes it's been useful to be able to prototype something in real-time while bouncing ideas off each other or if I'm trying to help them get something specific working in their code (but I don't have nearly as much familiarity with the other parts of what they're doing).