Well, I have always been touching horse on the rump, while sitting on them. Keeps him from being touchy or surprised when something unexpected does end up touching him.
I just have him standing still, twist slightly in the saddle, lay a hand on his back, spine, both sides of spine, top of hips, croups too. Something any horse needs to learn to be a BETTER horse when used. This is both English and Western horses.
Stuff happens in daily life, he has to be accepting of sudden, unexpected touching without throwing in a buck or crow-hopping with the surprised rider.
Two easy examples are a rider who may not be able to lift right leg, drags foot across his back while mounting. ANY bad reaction then is going to hurt someone! Another is the horse being “ticklish” when you wear a rain coat or longer slicker, that may rub on him while riding or even putting it on while horse stands still. Being halfway dressed on a long-skirted coat is going to hang up your arms, probably off balance as well. May not end well here either.
One I see commonly, is standing rider and horse, with rider twisted to look or talk to someone not close to them. One hand on horse rump to brace themselves, to view the sights or talk for a few minutes.
Horse needs to be accepting of being touched unexpectedly, not react poorly. So I think this rump touching is a training step that needs to be on the list for training any equine.