Fast leveling

If you're level 1 and they are level 90, due to the level-based xp sharing ratio you will get 1/91 of the xp (about 1.1%) and the level 90 character will get 90/91 of the xp (about 98.9%).

Think about actually being level 1 and killing everything in the beginning on act 1 normal on your own, solo. It's not too bad, you level up decently fast, right? Now imagine only getting ~1% of the xp (leveling taking almost 100x longer).

This "trick" (a level 1 being level boosted by a high level character) does work in Hell difficulty - the xp sharing ratio works the same, and the level 1 additionally suffers a massive xp penalty due to being vastly underleveled compared to the mobs, but you will still level up fast because of the sheer amount of xp each monster yields compared to how little it takes per level-up at the lowest levels. You can only pull it off (being level 1 in Hell) by being grushed. (see multiple other topics in the forum or other sites about the concept of grushes)