A lip on the front of a shelf can easily retain a tool box.
Since you don't need a bungie cord or other tie down, the setup looks cleaner and it's quicker to grab toolboxes since you aren't messing around with hooks and eyes.
If you have plywood shelves, you can quickly add a 3/4" or so wide plywood strip to them and get rid of the bungies.
If your work truck travels on very bumpy roads, this tip might not be for you.
