I'm rather early in Fallout 4, but I am far enough to have received my first power armor frame. I have affixed some equipment pieces to it and used it, but I also stumbled across an additional frame while wandering around. Can I do anything with it? It seems like I'd only ever need one frame. I don't believe that I can scrap it, and I haven't found any human companions, so I can't test if I can give them power armor. Is there any benefit beyond it looking nice outside of my garage?
Companions can wear Power Armor. Therefore, keeping multiple Power Armors allows you to give your extra Power Armor to your companions.
Companions can put on power armor if you direct them to it.
Also, if a settlement is under attack and there is unused power armor present, a defending settler will put it on automatically.
In either case, if you want the power armor back, talk to the wearer and there will be an option to ask them to remove it.