The screen retains the gravel. The gravel retains the formation. The gravel has to be sized to the formation sand; a pack that is too coarse lets sand through, and a pack that is too fine plugs with fines and cuts the rate. A gap in the pack, an empty section of annulus, is a path for sand.
A gravel pack is one sand-control method, alongside a standalone screen and a frac-pack. It does not stimulate the rock beyond removing the open annulus. A frac-pack places a short fracture and then packs it, and it is chosen when the well needs both sand control and a lower skin.