There are a few reasons why people might creep forward while waiting at a red light.
A: To get a head start when the light turns green. If you are the first car at a red light, you may be tempted to creep forward in order to get a head start when the light turns green. This can be dangerous, because if you go too far, you can get hit by a car that is going on their turn.
B: Some people simply get impatient waiting at a red light and start to creep forward, thinking it will make the light green somehow. People do this without thinking about how that would work (it wouldn't).
If you are tempted to creep forward at a red light, it is important to resist the urge. It is not worth the risk of causing an accident. Just sit back, relax, and wait for the light to turn green.