Piercings are one of the many ways to express yourself. An increasing number have opted to have their tongues pierced. This piercing, while offering the ability to express themselves, is easily hidden from the "business world" there by offering the wearer the best of both worlds.
People get their tongue pierced to make an image statement. It suggests that the person is brassy, daring, and cool. It might be a way to establish status in a social group or a way to show dedication to a subversion of conservative mores. There's an oral sex connotation.
People often pierce their tongue for aesthetic reasons - they think it looks cool or their friends have done it. Others believe it makes them perform sexual acts better.