Enemy AI usually can't use it
Which is dumb. These vents are always huge, convenient, blindingly obvious and easy to traverse. Why should they be a safe space for the protag? Why does everyone in the world except the player character have to carry an Idiot Ball the size of a watermelon? Why not simply have hallways where the enemy doesn't like to go?
plus it can connect multiple levels or rooms, sometimes vertically.
You're describing hallways and ladders.
And another thing. "Vents provide verticality" has been said by more than one person. Why the fuck?? Because climbing up the inside of a featureless, claustrophobic metal tube not designed for human traversal is somehow
easier than walking up some fucking stairs? You shouldn't be able to
turn around in an air vent, much less move effortlessly on any axis, yet
somehow this is the most convenient way to move between floors. If this made any fucking sense, people would be doing it every day in real life. But it doesn't make sense; it's "just how video games are", i.e. insultingly dumb.