For what?

In the first place it will probably avoid fraud with a simple substitution of the final time. Secondly, when watching the recording, it will be easy to determine which strategy is better suited for a certain part of the level. It will also allow you to study the races of other players in more detail.

At the moment I am writing an AHK script to automatically activate the timer at the start. Let me know in the comments if there is an easier way to do this, because I've only found this one.

For even more detailed examination of races I can offer the output of the pressed keys on the screen, which will also be an additional defence against cheaters.

It might help, but only in very rare cases. For example, if you need to slow down or if your speed still drops.

In this race https://www.speedrun.com/Rocket_Racing/runs/y2kgp06y, at 0:57-1:00, Wina applies a flip that allows him to land faster.