Zambo is correct; "Shear studs are welded to make the bondek formed slab and the steel beams a composite structure"
Forgive me if I'm simplifying too much, but it appears you are unsure of the definition of composite.
Basically with composite action the slab will be in compression and the steel beam in tension, the shear studs stop the tendency for sliding at the steel to concrete interface.
Without the shear studs the steel beam only would be carrying the load, and would therefore have to be larger.