A working Surgical Unit can make many areas of the game much easier. As noted above, most Surgical Units start the game in an incomplete condition, so you'll need these Activation Keys to give them full functionality. Blood-splattered Surgical Units are inoperative and cannot be activated nor used.
Since Activation Keys are fairly rare (only seven of them in the whole game), and they cannot be removed once installed, you should think carefully before installing one. Is this an area where you think you're going to need frequent healing? How near is the nearest working Surgical Unit behind you?