Sometimes people just use what's pre-installed. The last time I used Windows Media Player was in Vista. It was fine though.

I think I did associate mp3 files with media player for a bit. To prevent double clicking an mp3 from opening winamp and overwriting my playlist while I was maintaining it by hand. It would save on exit (maybe even on change as I don't recall losing changes to crashes, though they were rare), but if you loaded a different playlist (or a single mp3 "playlist"), then the previously loaded one was gone so hopefully you remembered to save.

Later on I just used the library and my star ratings for each song, plus some filters, to auto generate my playlist and no longer needed WMP.

I sometimes used the old one to play music that I didn’t want going into ITunes as it had an easier to use playlist than VLC did (at the time at least)