Some birthmarks are blood vessels near the skin (like a capillary hemangioma that grows for several months and then starts to recede). Some birthmarks are just moles, but it makes people feel better to call them birthmarks. Some are basically large freckles that don't mean anything unless you have several of them over a certain size (for example in neurofibromatosis). Some, like the “Mongolian spot” near the butt areas of some babies, show up in a ton of babies and slowly fade with age like the “stork bite” redness behind the neck or the "angel's kiss" between the eyes/forehead.
In the absence of a more sinister condition, they are just plain old unexplained luck or genetics (Mongolian spots appear more in Asians for example).