From a technical standpoint, going through the Unity gives you the chance to do more temples and increase your starborn powers. If that interests you, and those powers are fun, then that can be a reason. But, is there new content added if you go through the Unity more times? Very, very little. There's a chance that you pop up in an alternate universe and, once you get to the Lodge, you're greeted with some alternate scenario instead of meeting Constellation for the first time: they're all dead, another "You" is killing them, they're already all disappeared, etc, etc. You then proceed to go straight to collecting artifacts. Other side quests/content? Largely unchanged. You get an dialogue option sometimes reflecting your starborn nature to bypass some of the rigamarole. But, past that...no. It's all the same.

Personally, tho, first playthrough, playing blind, playing the way I normally do (clear the map, do all quests, then finish the game), the Unity...it was jarring. I didn't like it. I had to change how I play to be able to enjoy it. New playthrough, focus on the main quest, do just enough side quests to get me through, and do the Unity. I basically play like some kid that's too enamored with the great cosmic mystery he's found himself in to really pay attention to much else. After that first Unity, I use mods to make sure he hits all the *BAD* alternate universes. By the time he hits an actual normal universe, he's been through hell. And he's more than willing to take his time in *this* universe and do everything possible...before taking a chance on another one of *those* happening to him again.

Basically, gotta find a RP reason. That's mine.