... and that's the attraction.
This is "hardware as a service" so you don't have to put together your own hot-spec PC to find its then out of date a year later and watch it gather dust thereafter. You get to enjoy latest spec hardware, always. That, and you don't have to put up with all the faffing and fiddling with the operating system, patching, incompatibilities, custom ini files. That was the very reason I switched over to consoles in the first place, simply turn on and play.
You do need the games, though ...
... and so, linking your existing Steam account (which, if you play games then you're likely to have a Steam account and some games) to GeForce NOW allows you to play many (most?) of your catalogue straight away. You can also link your Ubisoft+ and Epic Games (and GOG, I think) if you've bought games there. So, this turns your physical purchases that you will have downloaded to play natively into a right to play also via subscription.
You can also try out Stadia (from Google) which will allow you to trigger a free month, I think ...
With Stadia (and GeForce NOW) you don't even need a controller as both will play well with keyboard/mouse input and Stadia kinda adjusts itself on the fly so that games that give key hints like Tomb Raider (press E to activate ... or X on a controller) will give the correct hint given your input device choice. Xbox Cloud doesn't as it assumes you have an Xbox controller. Stadia gives you a bunch of free games every month, which you claim and so long as you have asubscription you'll have access to them. There's also free weekends with AAA games.
Controller-wise, I have a latest PS4 controller which works will all the platforms I'm using: GeForce NOW, Stadia and Xbox Cloud. The Stadia controller is a nice piece of kit and will connect via a Chromecast, Google TV or even just a Chrome browser on any platform. Microsoft Xbox Cloud is best on Edge (no surprise) and gives the clarity boost on Windows and Mac, but not Linux.
So, a couple of platforms that folks can just get started with through a browser on pretty much any computer using keyboard and mouse, and some options for buying a controller like a PS4 or Xbox controller via, say, CEX or eBay for a cheaper alternative or new.
TLDR;
Stadia is fantastic but limited in games. You can play with a keyboard and mouse through a Chrome browser. Given the really excellent crystal-clear graphics, I do hope this continues and more games become available. This is a good choice if you have no games and want to try out cloud gaming.
GeForce NOW is fantastic, but you have to own the games. Again, play with keyboard and mouse via a browser or the application (Windows/Mac). This is a good choice if you have investment in online-purchase games (rather than physical media) to link your accounts and play the games you already own.
Xbox Cloud is still in beta and not so good graphically, but with the Game Pass Ultimate (£1 to trigger a trial month) you get access to 100+ games including a few AAA titles. You need a controller - Xbox or PS4 are good choices. This is likely to become the best option as the Game Pass Ultimate is a superb subscription, but the drip-feed of games into the cloud platform is slow ... they are still trying to see consoles, mind.