Why air compressors in engine rooms?

Usually for horns but can also come in handy for maintenance. Some older boats also had air windshield wipers

Yep, Horns, air tools and AirPush windshield wiper motors.
35 gallon tank with 3hp electric motor that runs from the gen-set or shore power and direct drive Detroit compressor while running without gen-set.

While running on shore power or gen-set, oil-less compressor in use, I have proper air filters installed so I run a hose over the side and can dive the boat on a Hookah Rig.

Once you start using a mini air ratchet, you will wonder how you ever worked with out an air tool.

Then there's Microphor toilet systems

Had to look it up. Ahh, Err, Uhh,,,
Needs air, yes.
But;; Not on my boat.

I forgot about fenders, rafts, inflatable toys, inflatable boats, charging Hynautic controls (80psi) and back blowing restricted thru-hull pick-ups.

Does not take much to get some task done if your not in a hurry.
I remember an old Roamer House Boat with a small DCv compressor (1 gallon tank). Worked like a champ.

I have been tinkering on bikes, cars/trucks and boats my whole life.
Every projects starts with the same question, Where's the air compressor ?

I'm sure once member Prospective thinks about all of these responses, He may ponder a compressor on his ship.

You convinced to bolt one in your ship yet?

Down here you just make a small cut under a gators skin and blow air in there. Puffs the whole skin off of the meat. Fastest way to skin the lizards.

Have a friend with a full chest freezer, G31 batteries, heavy patio furniture, mini fridge and now an air compressor all upstairs in and on his extended fly bridge.
So he ask; What's wrong with his NAIAD Stabilizers???

Most compressors don’t handle large inflatable toys very well because of the volume.

one of the best tool I keep on board is a Ryobi 18V compressor / inflator. It delivers high pressure for tires, hynautic tank, fenders etc as well as high volume low pressure for tubes and larger inflatables. Uses the same batteries as other Ryobi tools.