git log will only show you commits in your history. If you’re only ever working forwards, this will contain all the stuff you’ll ever need.

But if you’re rewriting history, like with a rebase or squash or something, or you’re deleting branches without merging them, then you can run into a situation where the official history of your branch doesn’t contain some of the commits that used to exist, and in fact still exist but are unlinked from anywhere. So reflog is the log of where you’ve been, even if where you’ve been isn’t in the official history anymore, so you can find your way back to previous states even if there isn’t otherwise a name for them.

If all you care about is your current history, git can use the dates of commits just fine to see where you were on Thursday without needing the reflog.