Why do you use Music Streaming services?

This question stems from a couple of questions asked by a (non-techie) friend the other day which were these:

Why would you pay a per monthly charge for music which you don't own? This would mean you would have to pay (Spotify/Google) forever, If/when you decide to stop you would lose all the music you listen to.

If you liked a song or album wouldn't you want to buy it anyway so you have security in still being able to listen to your favourite music without paying an ongoing fee?

I couldn't give a very good answer at the time so I thought I'd post it on here.

Do you use Spotify/Google etc. to listen to more music just because you're paying for it?

Is no one thinking about what their music situation might be in the future?

Or am I missing something blindingly obvious?