Some stone species are porous and will absorb water. These stone types are more susceptible to staining. By sealing the stone you make it both moisture and stain resistant. Notice I said stain resistant and not stain proof. Other stones are very hard and dense and hardly need to be sealed at all. The faster you remove the spill the less likely you will have a problem with staining.
Granite is like wood in the sense that not all granite is the same. To make the analogy to wood is to say that some stones are porous and less dense like a soft wood, pine as an example while other stones could be compared to oak. The later would be very dense and not nearly as porous.
As soon as you hear someone saying that granite will stain and needs constant maintenance you know that this is a generalization and only true for some species. In fact, by percentage it is very few stones species that this is true for.
Most granite slabs are resin treated with an epoxy or polyester resin before they are face polished. This is done to strengthen the stone, fill in minor blemishes and pits. This process also has the benefit of making the stone very moisture and stain resistant.
What stains stones?
If not resin treated or sealed, almost anything such as soap, wine, cooking oil, mayonnaise and other oily foods. When resin treated and or sealed, almost nothing can. Soap, yes liquid dish soap is the most common counter top item likely to stain a stone counter.
We have tested many common household counter top items and found that soy sauce, wine, vinegar, toothpaste, peanut butter, mayonnaise are all kept out rather easily while dish soap presents the biggest challenge. The very composition of what make soap effective as a cleaner is what makes it also very difficult to seal out of pores. Its chemistry is designed to get into microscopic crevices and clean them out. Often we do not think of soap as an aggressive product, but it can be.
Seal your counter once a year, wipe up spills as they happen and you should be able to enjoy your counter or many years to come. When you do get a stain, clean it with a poultice right away. Waiting allows it to penetrate and makes it more difficult to remove. Lots of information is accessible on this website to help you remove stains. The Marble Institute offer a lot of information regarding stain and poultice removal, See our links page.