1. My horses are worked in winter. If they qsweat, which they often do, then they need to be able to cool off enough that they don't get chills when I put them back into the paddock. Rugging keeps their coat short, which makes this easier. I have body clipped my main horse, and strip clipped my pony and a freinds pony to further help them in cooling and drying out after work in winter. It's a health concern.
2. Helps protect from bites/kicks from the other horses.
3. Canberra winters! It can be warm during the day and then that icy wind comes up that they can't get away from. A rug breaks the wind and really helps them keep warm.
4. laslt,y it is a little bit about looks. I compete all year round, so having a decent coat is important to me. I don't have stables, so rugging and clipping are the only ways I can achieve it.
If I had a horse that was a pasture ornament, I wouldn't rug all the time, but throw a canvas on for those cold Canberra nights.