It allows you to separate certain essentials (getting possibly important notifications, calls) from temptations that you want to avoid (twitter/X, instagram, tic tok, etc...)
What is the watch adding? Were you having trouble distinguishing calls from notifications before?
If I get a message in the middle of a meeting, I can figure out if it's just my mechanic saying my car is ready to be picked up. Or if it's a severe medical emergency. All this without bothering anyone else with audible alerts or getting distracted by any number of things on my phone.
If you have your own system, that's fine. I'm not saying this solution is for everyone. All I'm saying is that it's a useful solution for at least one person in this world.
The solution might not make sense to you, because you apparently don't have the issue. For some people, a phone making a sound in their pocket, ANY sound, or vibrating, or just merely existing, is an invitation to do some of that sweet sweet scrolling that releases the good chemicals.
A smartwatch (like the Apple Watch) is, at this moment, just smart enough to allow you to interact with simple yet "smart" (i.e. not phone/SMS) apps, like WhatsApp, maps, music, payments, tasks, exercise, but inconvenient enough so that procrastinating on your watch just isn't a thing.