One of the reasons that I’ve never mentioned this tip before is because, until recently, I hadn’t used this feature myself. Until I got the Apple Watch 3 with LTE, that is. I used to use the Watch as a way to control my music on my iPhone, but thanks to the device independence that comes with a stand-alone Cellular connection, I am now using it to listen to music with my AirPods on occasion, as well.
Unfortunately, the gift of Apple Music on the Watch hasn’t been bestowed on us quite yet (although it should be available soon). However, the ability to sync music of your choosing over to the Apple Watch for local playback has been a feature since watchOS 1, even if it didn’t have as strong of a use case as it does today. The Series 0, 1, and 2 Apple Watches have 2 GB of storage available for music storage, and the new Series 3 with LTE has been expanded to 8 GB. Note that the non-LTE version has the same 2 GB limit.