Is promise future needed ?

Question

We don't have callable function passed to thread returns value.
So, Std::ref is the one which helps

Other option is using promise which also help to provide us value out of thread function

I have used both the approach in below code.

Question 1 :
So, Why so much complication of promise and future when we can use std::ref easily

Question 2 :
Is there any other use case of future and promise or is it just unnecessary alternate?

Question 3 :
Also, future promise is complication as we have to join thread also which is blocking call
and future.get is also a blocking call.
https://sololearn.com/compiler-playground/cBMoWNzA2Fph/?ref=app