Yes, it treats apnea - but that carries over into other aspects of your life. Many people feel more alert during the day. Apnea affects your health in many ways, e.g. hypertension, heart disease, and so on.
For me: I’ve said in the past that when I started my CPAP therapy, I didn’t think I felt better - but when I did NOT use it, I felt worse - which is a conundrum. I do think I sleep better with the damn thing - but I still do not feel more energetic or alert.
To be fair, I have other sleep issues, including Restless Legs Syndrome and excessive daytime sleepiness. If ALL of those are treated correctly, I function. Not alert or chipper, but I can manage my life.
As far as a home sleep study, I can’t speak to that - my last study was in 2010 and was the full on wired-for-sound in a sleep lab.
Masks: Make sure you get to try a variety of them. I’ve been mostly using a specific nasal pillow mask, as it’s the least obtrusive - though it’s so lightweight that the weight of the hose pulls on it - so I have a clip (consisting of a Velcro OneWrap hooked onto a badge clip) to attach it to my nightshirt. I sometimes use a slightly different model.
My husband uses a “full face” mask (which is a misnomer - it covers the mouth and nose). I personally don’t know if I could tolerate that - as a lifelong asthmatic I tend to get twitchy about anything that even SEEMS like it would impair my breathing, even when I intellectually know it would not. There is also a nasal mask (which covers just the nose, but otherwise somewhat resembles the full face one).
You want to make sure the headgear is something you can manage easily when half asleep. You may also want to use a separate chin strap to help you keep your mouth closed (if you go for a nasal mask or nasal pillows).
I would definitely encourage you to insist on an APAP machine. When I first got one, I did so - because I had good reason to believe that the pressures they measured in the “titrating” session were not accurate. As it turned out, the measurement was reasonably accurate, and I stayed with that pressure for many years until my AHIs (and maximum pressure) began to creep up.
I will confess to tweaking my own pressure setting somewhat recently: when I had COVID, I couldn’t tolerate the machine at all; even after I started improving, the machine seemed to aggravate the coughing. I bumped my range down a couple of notches - and that was sufficient to get me back to using it. My readings have actually been okay since then, so I may not bump it back up.