Stone is naturally porous, some more than others.  If you have every wet an unsealed concrete driveway or sidewalk you have seen how liquids can soak into a stone like surface.  Any liquid spilled on an unsealed stone will slowly seep into the stone, unless the stone has been sealed or "resinated".  If the liquid is colored, can chemically interact with the stone or a trace element within the stone, or has the potential to attract dirt, it has the potential to stain your stone.  By sealing the stone you effectively seal these pores preventing the spill or liquid from soaking into the stone and forcing it to remain on the surface where it can be cleaned up and removed from the stone.