This theory attempted to predict the behavior of objects in a system with no outside forces. Descartes explicitly defined motion as simply the product of size and weight. His theory did not include mass and stated that speed, not velocity, was the property being conserved.
The main problem with conservation of motion is that not all variables are accounted for. Descartes had almost, in essence, stated the modern law for conservation of momentum, but he failed to consider the implications of mass and velocity in his formulation.
This notion that conservation of a few distinct properties does occur in the universe helped lay the foundation for the three main laws of conservation that are accepted today. This eventually revolutionized the fields of kinematics and classical mechanics, which are two branches of physics that deal with describing the behavior of objects.