Can someone explain to me what are wheels spacers, or disc brake wheels spacers?
And what are they used for, and preferred applications?

Many people use them for the looks. The spacers add more space between the wheels, wich mean the car is wider and functional, that means its more stable.

It also allows you to put on rims that were previously unable to fit on your car, due to offset.

downside is it wears down the wheel bearings faster