As someone who owns both and a PC as well, yes. Playing my steam library in bed is incredible. Also, if you have a larger library, it can be cheaper to just buy a Deck than re-buying everything on a different platform.
You'll be able to play any game that can be run on Proton (Valve's Linux compatibility layer). Technically, you can install windows on the deck and then run anything on that, but that's not supported out of the box.
Maybe. I have those two and a PC and I think the Deck is essential. But I do really like being able to seamlessly pick up my games on my main machine sometimes too. If I didn’t own a PC or a PC game library it might be less enticing but even then I still think it’s worth it. You may struggle deciding where to buy games though and you will probably end up building a gaming pc if you enjoy the Deck.