Doing anything even remotely active, you don't necessarily have your cell phone in a handy spot: jogging, biking, swimming, sailing, fishing, etc.
Why would I actually need a wristwatch for any of those activities? Competitive running or navigation on a sailboat out of sight of land maybe but fishing? I don't know where you fish but I don't really want to know the time when I go fishing. The whole point is to not worry about it. My philosophy on fishing is that if you need to bait the hook you are missing the point.
The ONLY thing I can think of that a watch makes sense for is if you need to carry some sensors in a compact way ala fitbit or if you are doing some very niche activities where knowing the time immediately is critical. Otherwise they are simply redundant.
You also can snag a quick look at your watch without getting caught - much harder to do with a smartphone, and much less socially awkward.
As opposed to looking at one of the several clocks that is almost always in view? Hell I have 3 on the dashboard of my truck put there by the manufacturer. As I type this I have 2 on my monitor, one on my smartphone, one on the wall and there is another just around the corner. I cannot fathom why I would bother lugging around yet another redundant device.
Smart phones also aren't very pretty.
Neither are most watches that are actually affordable. Personally I don't really like to brag about the size of my bank account via jewelry but that's just me.
People would probably wear something on their wrist even without the time-telling feature - that is sort of a bonus.
Women maybe. Not most men in this country.