Activity Tracker, Smart watch is the necessary thing or expensive knickknack?

Today on e-mail I received mailing from shop of electronics. Also it was surprised how I lagged behind life. With might and main Activity Trackers and Smart watches are on sale. Looked at them and understood nothing. Went to read and too little understood. Especially a difference between two these gadgets. I saw a difference in the prices at once, of course!


Here on a photo the most budgetary from Xiaomi. They consider calories, physical activity, even dream duration. And that - that there still... And yes, they show date and time. Notify on SMS. It is possible to float and they will not deteriorate.

Here the most popular brand and model of Fitbit. The price is 4 times higher, than Xiaomi. Well and the design differs. I did not notice other differences or did not understand.

Smarts watch are more expensive, they it is more by the size. But in what differences, I did not understand.
Perhaps who - nibud will prompt?
Perhaps you have such gadgets? Whether really they are necessary to the modern person?
In one response read that duration of service of them about 2 years, leave then new models.
Or perhaps without gadgets life is simpler?... What will you tell?