Great reviews for this app. Works with both PC and Xbox One. Get it for free!

https://www.microsoft.com/en-us/store/p/torrex-pro-torrent-downloader/9wzdncrdsgkg

Another one to go along with the Torrex Pro. Also works on both PC  and Xbox One.

https://www.microsoft.com/en-us/store/p/8-zip/9wzdncrfhwb8?activetab=pivot%3aoverviewtab

Offer already over?

It says $1.99 and $0.99 for me.

click on it and see what it says when it actually loads the Microsoft store app. 

Torrex Pro adds a media player and streaming to let you watch and listen while you download. 

Also I think the free version has adds but not sure. 

https://www.windowscentral.com/torrex-pro-adds-media-player-and-streaming-let-you-watch-while-you-download

I get why someone would want to use it on XBO given that it's a rather closed system, but why would anyone want to use an UWP app over a proper program on PC?

Easy to locate, (re)download, (re)install, and use. 

I've had Torrex Pro for maybe 3-4 years now and it's met all my needs across PC/Phone/XB. 

I can't speak for you, but that's why I use this particular UWP app over a proper program on PC. I'm sure someone can point to other better torrent clients they prefer but it comes down to personal preference. 

Not working. It's 2€ and 1€ in Store as well.

All of the reasons you mention are of very little use. The real benefit comes from not having to manually update anything (or hoping the publisher of the software has implemented their own system for automatic updates).

That said, not worth it to me in most cases since software outside Store is usually better, but it's still a valid point.

what is the difference between let say free utorrent?

I'm not sure what you mean? That's why I use Torrex Pro. Very little use to whom?