DT is used for making sure things that happen over time (eg. motion) take the same amount of time no matter what the framerate is. Eg if it is supposed to take 1 second for one object to move from A to B, with DT it will always take 1 second even if the user's computer is running the game at like 20fps.
Anything that happens instantly (eg. in 1 tick) such as running a check or teleporting an object doesn't require DT :)