It seems to me we don't have anything too rare here on Earth. They fan find their water, oxygen, nitrogen, etc much closer to home. Basically the same reason you don't travel across the world for a cup of coffee. We could potentially have a lot to gain from an advanced civilization -- specifically information. It seems unlikely to me that faster than light travel is common or economical even for aliens but maybe there's some chance of inter-galatic quantum communication. Maybe a handy blueprint on how to teraform local planets, cure diseases, efficiently produce endless amounts of energy, control & repair our environment, harvest resources from local asteroids & moons, track & divert foreign objects headed towards Earth, and so on.