I have a fitness tracker, which is sort of a smart watch lite.
It tracks my heart rate and through that can monitor my sleep, which I find interesting. It can also automatically pick up any workouts and log them in an app for me, which I don't currently use but have found that motivating in the past - I have ADHD which means I struggle with habit formation, but tangible feedback like "You ran 2 mins more this week" is helpful rather than something less clear like a vague sense of feeling more healthy.
It means that I can be a bit separate from my phone. I am quite prone to overusing my phone for the social media etc whereas the watch will alert me to things like text messages/whatsapp and missed calls. I can also set alarms on it, so I don't need my phone in the bedroom at night. I originally got it because I was starting a job where I had to leave my phone in a locker in the staff room and it was nice to be able to receive text messages, even though with my current watch I can't reply.
Also, it's a watch, that syncs the time to my phone so I don't have to change it for daylight savings etc. I can set reminders on it. I can set it to alert me if I've sat still for too long (since I have a toddler this never ever goes off XD but it's useful since my mind tends to wander and I can get sucked into things like MN for too long)
I can control music/podcasts playing on my phone with it which is useful if I'm out and about and don't want to get my phone out of my pocket especially if it's raining.
I can use it to locate my phone if I lose it.
It gives me a weather forecast much quicker than finding my phone and loading up the weather app and waiting for the ads to close etc - for example when I'm getting dressed I find this very useful.
Some people won't need both, I quite like having both. If I understand correctly, a smart watch does need a phone to connect to, it doesn't work on its own. But I remember thinking about 20 years ago that it would make sense if in the future people used their watch as a phone. You could perfectly easily use it to make calls, combined with a headset ideally, use voice to text or send voice notes in whatsapp, so just pair it with a very cheap basic phone that is kept in your bag or similar. I suppose the part I was missing back in 2002 was that in "the future" everyone's phone would not only be a phone, but would be a mini game centre, ebook reader, web browser, digital wallet, holder for bus pass, email reader, camera, video camera, photo storage etc etc etc. I like using my phone for all those things so I wouldn't want to do totally without it.