Having a few basic tools required to ascend a rope, set up a mechanical advantage pulley system, or cut a rope is a great starting point for just about any sort of rescue scenario, crevasse or rock, that you can think of.
Sure, you can improvise this with a prusik loop and a few biners, but having some relatively inexpensive and dedicated bits of rescue gear is a choice many climbers will make.
On my harness just about all the time, I keep a small locking biner with the following:
Wild Country Ropeman micro ascender
Tiny hardware store cheapo lockblade knife, with tape wrapped around it to prevent it from opening accidentally, and a short clip in loop attached (tape not shown in photo)
If I was feeling a little more flush, I might replace both the rope man and the pulley with a Petzl micro traxion. But those suckers now retail for $100+, so that's more than I want to spend on seldom-used rescue gear.
The knife is used to remove sun bleached ancient webbing from rap anchors (hopefully replaced with new material by you), to cut a cordelette for rap anchor material, and if needed, to cut away a runner or rope under tension in some sort of awful rescue rescue scenario where it might be your least-worst option. (See the Joe Simpson book “Touching the Void” for an epic story about this very thing.)
The Petzl Tibloc mini ascender could be another good choice here. But I’ve seen those sharp teeth shred the sheath of a rope, so I prefer the less aggressive teeth on the Ropeman.