This touches on relativity, as no object is truly stationary. The earth moves, our galaxy moves, and so on. Motion is usually stated in relation to a particular frame of reference, such as the surface of the earth. We consider that stationary, even though it truly is not, but it is a reference point that most all can relate to, and agree with, as most of us live here.
The reason why reference points that are stationary are often used to determine whether an object is in motion is because they are still in motion. To determine motion for an object moving on or near earth's surface, a reference point such as a tree or building is used.