It does not prevent one diving into the spoilboard, just limits the amount of damage. I have found that sometimes care is not taken in setting up the toolpath on how the machine zero's on the Z axis. When setting up the material in the cam software, persons sometimes overlook where the Z is zeroing (top of material, or top of the spoilboard).
It is some times confusing to some who are just starting out, that the Z homes reverse to the other axes and incorrectly apply how the machine zero's in their toolpath can inadvertly cause them to cut into the spoilboard.
I sure helped me in the beginning. Subsequently have just left the limit in place, just in case.