Mulches, when applied at the right time, can be very good for weed control and prevention. The important thing is timing: if mulches are applied at a time when weed seeds are fully dormant, they will prevent any exposure of the seed to sunlight, which is for many weed seeds the primary signal that causes them to start growing.
Mulches are most commonly applied in spring for this purpose, but many biennial and perennial weeds actually germinate in autumn, so some gardeners will mulch twice per year, or even add mulches whenever they pull weeds or otherwise disturb the soil.
Mulches are sometimes applied in combination with a pre-emergent. For organic gardeners, the pre-emergent used is corn gluten meal, which is applied under the mulch.