An object - a player or npc or whatever - has a position, direction and speed and such.
Before drawing each frame, we want to compute the current position of each object.
How do you do this?
If you take the elapsed time since last frame, you run the very real risk of the game running at the wrong speed if the frame-rate is very fast and the clock precision poor (as it often is on some platforms).
And how can you incorporate player controls such as the player wanting to go left or right (e.g. in a simple asteroids clone without ship inertia)?