Because it's probably not just you and even small tasks can be useful to know the status and history of.
Boss: So what all did you do this year, including things that I wasn't directly involved with (like I've done in the past, working as support or internal "consulting" for other projects)
You: Well, I don't know, I didn't track it.
That's not the answer you want to give. Similarly, unless it's a solo project you'll want to be able to answer questions from colleagues. Since not every piece of data shows up in a git repo or JIRA tasks, your own log can be very helpful. Or just for yourself, "Wait, did I actually finish that thing or did I just scratch it off the 'todo' list?"