Because sleep tracking and an alarm that doesn’t disturb your partner are two of the best features.

I have a different sleep tracker and me and my partner use a wake light, so I don't find these things relevant to my use case. A quick poll of my friends is similar, although one uses the sleep tracker and charges while getting ready in the morning.

The Garmin watches have Pulse Oximeters in them that tell your your 02 saturation while you sleep, breaths per minutes, and use that data to calculate all sorts of metrics for you.