So, why would people do that?
One answer is that we're incredibly social animals. You have to keep in mind that the self is not a thing. It's an event. When you're in deep sleep, the self doesn't really exist. The neurochemistry for being a self is not there. So from this point of view, we feel most real when other people are affirming us, and reassuring us, and reinforcing our identity. In all the social rituals you go through, like saying "Hi, how are you? Fine, how are you?" You don't really expect to hear any personal information. It's really just a confirmation that you both exist and acknowledge each other. So in a way, tattoos function in that sort of fashion. They bring attention to you and make you feel real, even if the attention is making you feel like a member of a huge group. A tattoo tells you that you are one of, as it were, a tribe of tattooed folks that are really beautiful and significant. You may even share symbols with somebody else! And at the same time, because of the branding phenomenon, it makes you feel like you're smarter than the next guy who doesn't know enough to buy your particular product, or your particular fashion.