I'm not complaining at all about having an on-screen timer for future submissions, but I just want to know why would it be needed? I will indeed insert an on screen-timer in the future, but is it needed for some sort of accurate timing thing? Is it somehow easier if it's on screen?

On screen timers are a way to improve the validity of runs. They are harder to fabricate in general principle, and also help with accuracy and proofcalling. If you need timers, you can check out the guide in the Java Edition Minecraft pages. It is highly encouraged to have visible on screen timers, especially for WR runs.