Since the dawn of recorded time, humans have been compelled by the stars. We've written constant meaning onto the cosmos — life, death, the gods, the goddesses. We've made the stars our way of navigating the seas, of finding our way home. So of course, we also tattoo these constellations and their various meanings onto our bodies. It acts, to some degree, as a way to always remember these bright beacons, even when it's daylight.
Zodiac symbols, of course, are some of the more commonly known constellations. Taurus as the bull, Gemini as the twins. A Zodiac tattoo is a double whammy if you incorporate the constellation into it.
Our futures may or may not be written in the stars, but one thing is certain: these stars are certainly written on these people.