Natural resources
"You cannot yet appreciate how beautiful your world is, how much it possesses and what a rare jewel it is in a Greater Community of barren worlds and empty space. Worlds such as yours are rare indeed. Most places in the Greater Community that are inhabited now have been colonized, and technology has made this possible. But worlds such as yours where life has evolved naturally, without the aid of technology, are far more rare than you might realize. Others take great notice of this, of course, for the biological resources of your world have been used by several races for millennia. It is considered a storehouse for some. And yet the development of human culture and dangerous weapons and the deterioration of these resources have caused the alien Intervention." [1]