Why is offline music useful in a smartwatch?

For starts, if you prefer not to take your phone with you? I rarely carry my phone when running

That's not to say I'd be particularly interested in an LTE-enabled Garmin either, though. I like to run to be away from my mobile phone, to escape all of that connected technology. And while I may not mind carrying the extra weight of a phone on a long run, if I am doing any kind of session it really doesn't make sense to have more weight on you than is necessary.

I never carry a phone during my runs, long or short. This feature allows me to not have to worry about carrying a separate MP3 player, so this is a very nice and useful feature for me.

I hate taking my phone in an armband when cycling or doing a long run. It itches and makes the gait uneven at times. Using music from my Garmin is perfect and one of the main reasons I upgraded from my Fenix 3 hr.

As the others have said, it's for people who don't want to carry their phone. You can still control the phone's music with non-music watches. If you don't need the maps or battery life of the 945, you could probably get by with a less expensive model (245, 645, 735XT, 935).

the question is - why isn't it? :-) (getting philosophical...)

To not run with your phone? Phones are heavy. Run without it one time and you won't EVER want to bring it with you again. It's a total pain. I do still require my AW because I need LTE for emergencies (hint hint garmin get LTE).

Yes, that's the main reason I like to take my phone, for emergencies. But I will try not to take it in my future runs. I received this watch as a gift, and I am trying to make good use of its features.

I get that. I wear a watch on each wrist because I need that AW for emergencies. I also do tests to compare them so I would be wearing it anyway:)

I never used the feature either because I always take my phone everywhere I go and playing music from the watch drains the battery pretty fast. And then I started going to the sauna again and now I know why I need offline music in my watch ;)
You are obviously not allowed to bring a phone with camera to the sauna but you can bring a watch and some bluetooth headphones (AfterShokz Aeropex) and now I have nice music while relaxing.
Could have taken an old mp3 player instead but the watch I have on me anyways (nice to know your heartrate and HRV in the sauna, pretty high values for just sitting around) and the sync with spotify/deezer keeps the mucis fresh.