Basically, since you already have an Xbox, Microsoft Xbox PC gaming venture is pretty great for if you enjoy that ecosystem, and regularly use the machine. You can get many games once on Xbox or PC and also own the other version on PC or Xbox respectively gratis. You get the same ecosystem, achievements syncing, friends etc and soon (or already) lots of UWP apps (though not always identical between stores as some might not be allowed on the Xbox console store.
However, Microsoft's Xbox version of PC gaming is very much just that, the ecosystem is more closed, less interoperable and games ultimately rely on Microsoft services to self serve that bubble. You practically get a more flexible console version of said game. If Microsoft is to believed, they also seem keen to have updates be in lock step between the to systems, which throws away much of the benefits of the PC gaming space in the first place.
You don't really need your Xbox unless you are keen to play exclusives not found on the PC Xbox store, regularly use that machine / haven't got a PC conducive with the same spot the Xbox is in your house (eg your PC is a huge tower / can't get an HDMI cable there etc.), or if you need the ease of use you expect of the console.
You could easily use the Xbox PC ecosystem as its own bubble without the console, but also utilise the myriad of stores, games, services and openness of the PC in general for gaming. Many stores like Steam GOG and Humble will grant you win / Mac / Linux versions of games when you buy and as they become available, so you have a bit of future proofing there should you end up on a different OS later. Prices are much more competitive, variety of games is ridiculous, many features or services like Steam and all manner of hardware / peripherals and services can be interoperable, modifiable and easily shared around.
Also join the SteamGAF community thread for some spare and gift games across a variety of stores and cool deals.
I think you mean download swat 4 from gog and don't buy anything else because that would be unnecessary
Both are pretty damn good.