Why do granite counter tops need to be sealed?

Granite is real stone, and as such it is naturally porous. Microorganisms can grow in the pores and contaminate food on the surface. Makers of granite countertops therefore provide or recommend sealants, usually a form of spray silicone, to minimize the natural porosity of the stone. It should be noted, however, that natural stone countertops are not certified in the United States for use in commercial food processing because of the porosity/microorganism problem. There is another form of "granite" top called "engineered stone" or just "quartz" which is basically a ground up granite mixed with resin. These materials, which go by many brand names, do not require sealing because they are permanently sealed with resin, but they have their own quirks. The main thing for most people is esoteric: if you love the natural beauty of real stone, go for it, and seal it according to directions and keep it scrupulously clean.