You would have to check your local laws, but I don't know of anywhere where it is a requirement for a trailer.
I can't really think of a reason for them, unless you do a lot of nite backing. In the rare case where I need
light back there, I set out a flashlight, or even turn on the rear cabin lights and open the curtains. The side
light outside by the door can be used as a runway marker on my trailer, I have used it before too.