Whether it needs it, how often, and what sealing does not do.
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Because granite is a siliceous rock with very little porosity and marble is calcitic with open pores. A penetrating sealer slows liquid getting into those pores. It does not stop acid attacking the surface, which is the mistake that costs money.
Before the surface goes into service, after it has been fully cured and cleaned, and never over an acid-etched area. A sealer applied over an etch seals the etch and makes the dull mark permanent.
On a floor, a wet room or a counter, yes: an impregnating sealer, on a schedule. It slows a spill getting into the pores and it makes the stone more stain-resistant. It does not make the stone acid-proof, and nothing does.
Ordered by how many questions each one holds.