Its function is to prove the value of the Mark's wielder. This is why the enemies inside can only be seen when the wielder activates his Gathering ability. The mechanisms also only react to the Mark of the Outsider. It acts as some kind of trial, and if it's successfully passed it "awakes"
the Island and grants the wielder of the Power the power to control it.
In the Realm you'll face the Shade as the boss of the place.
You can find weapons that enhance the abilities of the wielder of the Outsider's power like the Lapis Lazuli, and it is easy to collect Light of the Outsider, meaning that it was specially designed for it.