On a Jeep with leaf springs you will find that the plates that the u-bolts pass through are some of the lowest items under the Jeep and damage to the u-bolts is very likely without some protection. There are many varieties of plates and all do a good job. Essentially these plates attach to the u-bolt plate and provide a skid to allow the Jeep to slide over an obstacle while protecting the bolts/nuts.
On the jeepfan.com 1978 CJ-5 we’ve been running skids made by Rock Equipment and their design is a little different than most. These use tabs on the sides to bolt the plates which allows the skid to be solid.
skid installed 
exposed bolts and nuts
Since we were swapping in a new set of Old Man Emu springs we installed new u-bolts and took some time to show the details of these skids.
marking the bolts 
cutting 
ready
You can see how far the bolts will stick down and how they are very vulnerable when not trimmed. Some people will tighten everything up and cut the bolts off when installed, we find it easier to mark them and cut them when removed. Clean up the threads before installation to avoid damage. When the u-bolts are tight the skid is installed using the two side bolts.
skid installed
You can see how the skid has taken a beating over the years of trail use, imagine that the bolts and/or nuts would have looked like without them.