Much like Ben is saying, the wood fibers are more resilient in an end grain board against the chopping motion of the knife. Also, this style is somewhat easier on the blades (keeps them sharper longer) than something like an edge grain alignment.
Another comparison besides the toothpicks or skewers I’ve heard is that of cutting the bristles on a brush. If you try and cut the bristles by chopping from end (the part that touches your head) to end (the part that secures the bristles to the handle), you’re not going to do very much damage. But, if you try to cut across the bristles, you will cut them in half a lot easier.
Of course, you can use edge grain or any pattern you wish and the type of wood does make a difference, but end grain is the better known style for blade life longevity and will almost always be preferred because of that.
-- "Well, at least we can still use it as firewood... maybe." - Doss
