I don't have a smart watch myself but several of my coworkers do, the killer app for them seems to be two factor authentication codes; we've got like 10 different accounts at work that need 2fa and instead of pulling out their phones they just use the watch.

It's a lightweight music player, ideal for fitness. That means that it would be a great replacement for the recently discontinued iPod Nano.

At the moment it's a bit pricy to buy one just for that use but maybe one day it'll be cheap enough that people will feel comfortable buying it for that feature alone.

I use the Modular watchface, and put things like other timezones on it, and a quick one-tap to timer. (great for cooking, or medicines, or Pomodoro, etc)

If you're a MacOS user, you can auto-unlock your computer with it (sounds scary even typing it, but it's damned handy)

A quick swipe up and you can use it to "ping" your iPhone (phone makes loud sonar sound) in case you can't remember where you put it.

If I'm on other side of house, and phone rings, I can actually answer call with watch and talk Dick Tracy style :-) (people tell me it sounds pretty clear)

Notifications and fitness tracking are the primary use cases form me. Those alone are enough but I also use mine for Apple Pay. It's a way better user experience paying with the watch than the phone.

It's also a great alarm clock if you wear it to bed since it vibrates silently. No more waking my wife up at 3:00AM when I need to catch an early morning flight.

I used a Fitbit for years and was reasonably happy with it. But the reality is that the Fitbit was cheaply made and not especially attractive. If you're already spending $100+ on a fitness tracker that is going to likely break in a year, it isn't that much of a stretch to spend a little more to get a really nice Apple Watch that lasts longer and looks nice when you're dressed up.

If you have toddlers or babies, the quick access o the timer is a killer app. 5 minutes to boil pacifiers. 3 minutes for timeout because you're 3 years old. 10 minutes until we turn off that TV show.

Notifications and fitness tracking is more than enough. My phone hasn't had the ringer on in over 3 years now. At first I had a Pebble which is pretty much just notifications. Now I have a Gear S3 and now I'm using the fitness tracking all the time.

With the new CPUs in the Series 1 & 2 watches I see uses in some businesses like what Google is trying with Glass.

I work for a medical billing company owned by a physician group, with deeper integration into hospital systems than the demographic-only feeds we get to stuff like labs and monitors would help us better enforce our quality programs in real-time instead of after-the-fact when coding.

I built an app for it to track my weight lifting (not aware of a Fitbit or Garmin that does that).

https://www.rikerapp.com