Adding to what pentode said, it's so your antenna will have a good match (good SWR).
Wherever you mount your antenna, it has to have a solid continuous "plane" of metal under it for your radio signal to travel correctly. Obviously the best plane will be the expanse of sheetmetal that is the center of your roof. But when the antenna is mounted onto some other separate piece of metal that is smaller than the car roof or body (your toolbox) means that piece of metal (toolbox) will want to try to become it's own ground plane if it is not electrically bonded to the body of your truck.
The toolbox itself is much too small to function as an effective ground plane at 27MHz, so is why it needs to be grounded to your (larger) truck body so that it can become part of the bigger ground plane that is your truck.
Here's some further reading on this just so you're not having to take my word for it:
strykerradios.com
Following a few key suggestions and tips will dramatically increase the chance of a successful, well-performing CB installation.
www.rightchannelradios.com