Can someone please explain, why use Matrix while there's XMPP ?
There's the least common denominator XMPP, with a lot of key functions under optional XEPs. Thus the user experience is very uneven across clients and servers. Matrix has a more defined set of important features, so you could expect that conforming clients all implement them uniformly, without surprises.
That said, I can't claim XMPP clients are any better.