Those aren't screws, they're called standoffs. Their purpose is to elevate the motherboard from the case so that there is a gap between the back of the motherboard and the metal frame of the case.
Standoffs are usually hexagonal and are screwed in to the case itself which allows for them to be removed but some cheap cases may have them welded on directly
the reason why you need them is because of how motherboards are made if the whole of the motherboard touched the case it would cause errors at best and be fried/damaged at worst
my first pc in 2000 had these if they still have them more than 10 years later, don't you think you need them?