Text notifications is a big one for me as well. Same with being able to see who is calling at a quick glance. It lets me know if I need to dig my phone out of my pocket or figure out where I laid it down. If I can't figure it out I can just answer the call from the watch in a pinch
I also have come to really like the Home app for unlocking/ locking my Schlage sense lock. I don't have to carry a key and can just unlock it with my watch. Handy if my hands are full or just returning from a walk with the dogs or a bike ride.
Ditto with the Nest app for the thermostat. I like to turn it down low when I will be gone at work or away from the home for awhile.
A quick glance at my appointments for the day and what I have up next
Timer as mentioned above
Quick access to Siri when driving, walking, riding for a quick memo or reminder.
I use the Tile app, when I all too often, can't remember where I or my wife placed the keys
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