What does the FixedUpdate loop do? Why do developers use it?

Remember we are talking here about Unity. And FixedUpdate is used much more often than the Update there. And you can call it any number of times each frame in case the frame rate is low and you might not end up calling it if the frame rate is too high, The Physics calculations and the updates do occur a second after the fixedUpdate. And when you are applying the movement calculations with it, you are not required to multiply the values by the deltaTime Time.

Why it is called and the developer uses it?

It is called on a reliable timer and is not dependent on the frame rate.

To understand its usage you need to understand Update and LastUpdate.

Update:

And we call Update one time per frame. And for updating the frame it is the chief Workhorse function.

LateUpdate:

And this is called one time each frame, and after we finish Update.

Explanation:

Please find the details in the Answer section.