Interactions with smartphones generally are of two types. Type one is full interactivity, you're using the phone to "do something" that takes at least a minute, maybe many minutes. Type two is reading a notification, which often takes a few seconds and often isn't immediately actionable. For reading notifications smart watches are perfect, because they reduce the friction of reading notifications significantly. You don't have to take your phone out of your pocket, you don't have to unlock it, you don't have to pull up the notification, you don't have to put your phone back. In situations where you are likely to receive a lot of notifications, which is increasingly common for many people these days, it makes it trivial to keep caught up on notifications even if you're busy doing other things. Granted, that's not always what you want (though it's also trivial to put a smart watch away or on mute) but when it is it can be anywhere from highly useful to a lifesaver.
For me, I started using a smart watch exclusively while working events where me and my team would coordinate using group chat. It wouldn't be uncommon to receive hundreds of messages per day, and sometimes dozens over a few minutes. The vast majority of which wouldn't require a response, but being caught up on what was going on would be critical. And often everyone would have their hands busy doing something else, so being able to just look at your watch to read a message is an amazing improvement.
I started wearing my pebble every day maybe two months ago just as an experiment (after spending something like 2 decades not wearing a watch). So far it's been a success. Being able to stay caught up on my messages with little friction is helpful, and surprisingly having a watch is actually useful fairly often as well.