Here's why:

* Motion is relative: There is no absolute reference point in the universe. Everything is in motion relative to something else.

* Example: You are sitting in a chair, seemingly stationary. But you are moving with the Earth as it rotates on its axis and revolves around the Sun.

Therefore, to describe motion accurately, you need to specify a frame of reference. This is a point or system of points that you consider to be stationary, and you then describe the motion of other objects relative to that frame.