It's good to know what marble is in order to understand how it behaves. It's a metamorphic stone, which is formed when sediment crystallizes under great heat or pressure to form hard rock. Marble is not the hardest of the these stones, however, making it porous and therefore susceptible to staining.
Marble is also formed out of calcium carbonate, giving it a chemical structure that reacts easily with acids and leads to etching on the surface. Due to all this, marble requires some maintenance to keep it pristine, though many prefer the gradually aged surface with etches and stains that blend into the grey veins over time.
Because unpolished or honed marble has open pores, it is important to seal it if you want to limit stains over time. But, think of marble as the jeans of countertops — they will work better and better while wearing and aging gracefully, giving them a unique and organic character. Just be sure that you care for it as it ages.