I like backing horses out for a couple reasons. One is that we had a 2-horse straight load for many years. With that style trailer, backing out is the horse's only option since there is no room to turn around. For horses that have never had to take that step down into the unknown, backing out is very difficult.
I once trained a horse for someone where the horse had never had to back out of a trailer. The first time I took her somewhere, she was too frightened to back out. It took me quite a while to convince her that it was ok. When we returned to my place, she still didn't want to back out, so rather than getting into a power struggle with her, I simply parked in the middle of my fenced pasture, untied her and opened all the doors wide. She stood in that trailer most of the day. It wasn't until suppertime that she finally got up the nerve to back out. Once she had done it on her own, future trailer trips went smoothly. She just needed that confidence booster.
The second reason is just that it is a challenge for the horse. Horses have very poor depth perception and, if their head is straight, they can't see directly behind their tail or directly under their nose. They try to judge the depth of something by raising and lowering their head. They adapt for those blind spots by tipping their head from side to side. Backing off a trailer is an act of faith and trust, just because they really can't see where they are stepping or how big of a drop off that step is. It truly is a leap of faith, a step into the darkness for them. The horse needs to/learns to trust their handler through situations like that.
Circle driving is a great exercise because it teaches/improves the horse's ability to lead well and to "latch on" to the handler. The horse learns to move with you. It really helps reduce any tendency of the horse to pull back when asked to go forward.
And finally, Xoe's been having way to much fun playing with the fencing right outside her pen. That particular section of fence is not electrified (on purpose, since it is right next to the yard and I don't want to shock all the little kids that come here). I wanted Xoe to learn some respect for fences, so I turned her loose in the main horse pen, which does have electricity flowing through the back section. The first thing Xoe did was to try to nibble the grass outside the fence. It didn't take long for her to get zapped! She ran and bucked and stayed away for a while, but the grass was too much of a temptation. The video below shows her second attempt.