There are three basic ways to travel back in time. There is the “do whatever you want” method, like in Back to the Future, which is the least logically consistent.
Then there is the closed loop, like in Harry Potter, which makes lots of sense, but allows for some paradoxes and negates free will. Also, alternate timelines can’t exist.
The only paradox-free way to travel back in time is to travel to a parallel universe that has the same history up to a certain point. Since Star Trek already has parallel universes and wants to have alternate timelines, while trying to have science that makes sense, this is the way to go.