Is that really just sugar? As I understand it most orange juice is concentrated before transport then rehydrated.

It's not just sugar. There's the occasional vitamin in there, probably hunkered down in a bunker to hide from all the roving hordes of monosaccharides, like a sort of kindergarten health class film strip version of I Am Legend.

But, as far as its culinary purpose goes, I suspect it ends to end up in processed foods because it's an additive you can use to sweeten the food while still being able to prominently claim "no sugar added" on the front of the box. I think legally you do have to cross your fingers behind your back while you do it, but it's not enforced.

I wouldn't be surprised if it's a trick that even works well on people who like to hang out on the internet talking about how fructose is the devil.

Pear juice doesn't taste like much but sugar water, so it's often used as a sweetener in things that already contain fruit.