Starseeds claim to be here for many diverse reasons, ranging from to save the planet from the horrible polluting people to help human beings ascend and become 4th dimentional.
Active Star Seeds do not fade to black upon removal from their hosts, and are sources of great power. As far as we know, only a planet's Senshi possess these. It is possible to live without a Star Seed, if you are given strong enough magic (as in Galaxia's bracelets). Active Star Seeds can apparently also take on different forms; Galaxia's Star Seed became a young girl who called herself ChibiChibi, for example.