I believe that is called a "catastrophe" fuse so that if a serious short to ground happened in something unfused, like the main line for example it would blow.
Many I have seen are cartridge fuses in old ceramic type holders. The rivets holding them together overheat making for poor contact and no 12 volt
power in the trailer.
How necessary they are is a matter of opinion I guess. I have eliminated them in several instances to get some one going who lost their 12 volts on the road