A light cover of soil or mulch will retain moisture and keep grass seed from being lost to the elements, but sand needs to be mixed with other materials to be a protective cover. Sand is used for sowing grass seed.
Grass Seed Protection
Freshly sowed grass seed can be blown away by wind or eaten by birds if it's exposed on the surface. Keeping the area moist until the seed germinates will prevent seed loss. If it can't be kept moist, cover the seed with a light layer of soil or mulch to keep it in place and retain moisture. The fine grains of sand need to be mixed with heavier material to be effective protection for grass seed.
Grass Seed and Sand
Mixing grass seed with moist sand helps sow the seeds evenly, Once the sand dries, much of it will disappear into the ground or blow away.
Sand for Leveling
Sand can be used to fill low spots in the lawn, by raking it in and pushing it down to soil level with a broom. Too much sand can bring weed seeds, pests and diseases to the lawn, so it's best to use it sparingly and not often.