Rust (and the OS) give you the precise time of the moment you call that function. However, that's not what you want.
Your game systems are run by Bevy's parallel scheduler, which means that they could be called at vastly different instants every frame! This will result in inconsistent / jittery timings and make your game misbehave or look stuttery.
Time gives you timing information that is consistent
throughout the frame update cycle. It is intended to be used for game logic.
This is not Bevy-specific, but applies to game development in general. Always get your time from your game engine, not from your programming language or operating system.