Concrete cracks - it's not a question of if, but when it will crack.
Control joints are placed in concrete to try to control where cracks appear and prevent them from randomly forming across the slab. Control joints are different than expansion joints, which are between two different slabs or two different materials.
If a repair material is placed over a joint or even a small crack, that crack will almost always transfer through the repair and become visible again. These photos show a typical garage floor with hand cut joints using a jointing trowel. If these joints were not in this slab there would be random cracking, but the photos show the cracks run along the joints and are not noticeable unless looking for them.