I have one on either side of each door, they support the side tent rings, and give me a secure tie down for boats on the roof.
http://www.instructables.com/file/FFR24IAH67NSO05 I can't use rings in the frame as mine is 5' wide on a 4' wide trailer. I also added one to each side of the back near the bottom, they anchor the roof I made to cover the tear for the winter, and give me a good place to tie an easy up to.
