If you already have a smartphone, the story is a bit different, I agree.
If you want a device that does a lot more, that may also be OK. But it makes sense only when you would also use all those features. If I would just like something that monitors my fitness, I would not be tempted to buy a device that can do a lot of other things as well. If I am looking for a chair, I will not buy a car, just because it is four chairs with a whole lot of extra stuff.
And I would not like to take the time to get acquainted with a smart phone, while I could master a fitness tracker in a few minutes. But then again, that is just personal.