Marble is mostly calcium carbonate. That is what gives Calacatta and Carrara their glow, and it is also why a squeeze of lemon leaves a dull mark within minutes. The acid is not staining the stone, it is dissolving a thin layer of it. That mark is called an etch, and no amount of wiping removes it.
Marble is also porous. Oils, red wine and turmeric can soak into it before anyone notices the spill, and once they are in the stone they are very hard to get back out.
A sealer helps with the second problem but not the first. It slows liquids soaking in, but acid still reaches the surface and still etches it. We explain the difference in detail in sealer vs protection film.
No film
Stone Shield film

