In the case of the shelves in the picture under the windows, the top also serves as a workbench. You need a toe space for your toes, just like in a kitchen or bath vanity. Some garages are at ground level and may be prone to flooding in extreme weather conditions, not to mention dripping cars, gas fumes, etc.. Shelves or cabinets are just as strong and weight bearing on a toe kick as they would be on the floor, since the toe kick is on the floor. Toe kicks can also be fit to uneven floors so the cabinets are level.
Cabinets and shelves that may be hung from the wall, rather than sit on the floor, are weight rated and designed for that type of installation. You may want to check the stated capacity before loading 20 engine blocks on them. Unfinished bare concrete is permeable and on ground that may contain water. Usually, you don't want anything in contact or close proximity to that potential moisture source. Tape a piece of plastic sheeting to a concrete floor and check it in the morning, there will likely be condensation under the plastic in many garages. Closed in bottoms in these situations can be a great breeding ground for everything that likes damp, dark places. You can't get in there to clean up without moving the cabinets or shelves. Just look behind your kitchen stove to see the nasty stuff that can accumulate where you don't often go.