Gravity is what allows you to walk on the floor, on Earth. This is due to what is called the force of attraction. Forget all the laws of physics in games of gravity! You will be able to change everything and walk on the ceiling.
Changing gravity can help you to avoid many traps or to collect high placed objects. In Gravity Duck 1, 2 and 3, for example, you have to help the duck to collect all items to open the door at every level.
If you change the gravity at the wrong time, that could make you fail the level you are doing!