Why do birthmarks?

I had a shower thought about me having a small birthmark and how my best friend does not, and knew that some people have them pretty large. Some are light and some are darker Which led me to ask myself, how do birthmarks happen? What makes someone have a birthmark or not? Are their placement significant in any way or are they some kind of natural unique identification like fingerprints? Does everyone have them and if they "don't", do in fact have them but the same color as your skin like the invisible stripes on the human's body?