Not the person you asked but for me, it's because Youtube Music actually has the music I care about (fan edits of songs, compilations, mixes, etc). Then I also get to watch the videos when I want to or just listen to the music when on the move. These two, but especially the first, are the big reasons I use youtube music.

I did it because why pay for Spotify and for YouTube music if I was going to pay for YouTube premium anyways. The one thing it is missing is when you are at a party on Spotify can have a shared playlist where people can just add songs to it from their own phones. But YouTube can’t do that. It also won’t crossfade songs which Spotify does. But I don’t feel like I’m missing out because of those reasons.

Lower price and just wanted to try. Forced myself to use it, started creating new playlists, listen to the radio created based on songs etc.

Now my YouTube Music is more used by the kids and the family than Spotify

You get YouTube premium and YouTube music with one subscription. For me it’s a no brainer, I’ve had it for years