Hiding gets you out of sight. Stealth is what lets you move afterwards, and the skill directly sets how many steps you manage before you are revealed. That makes this one of the few skills where a higher cap changes what you can physically do rather than just improving a percentage.
At 100 you cross a room. At 120 you cross an area — through a spawn, past a player camp, or into position for a disarm without the run-up giving you away first.
It is core to thieves, ninjas and any scout character whose job is to arrive somewhere without being seen, and useful on a treasure hunter who would rather walk past a spawn than fight through it.
Legendary powerscroll: cap raised from 100 to 120 on one character, consumed when read. Train the skill up to the new cap as normal afterwards.