When you buy meat from a butcher, it's probably been sitting there for a while (the meat, not the butcher). Now, your meat is almost certainly still safe, but that doesn't mean that little pools of various natural juices and other items haven't collected on its surface; such a thing is natural for a piece of meat sitting on ice, naturally filled with fluids (and perhaps pumped). Those natural juices and other fluids may not taste so good, and nobody wants to bite into a piece of meat and find a sudden pocket of bitter or otherwise distasteful flavor. If you simply rinse, then pat dry, your meat, you'll ensure that these surfact fluids are washed away and that you get the best flavor you can from the product.