Reach is kinda like earth, ie:rocks mountains,water, tree's, but mostly mountains, the covenant are after reach because they think all humans are a disgrace to their religion, as of the spartans are made on the planet and that humans occupy the planet , they want to destroy the humans and Reach seems to be in the way where the covenant are going to attack, and that reach was a important base planet with information to get to earth , and the planet had armies on it that could have reinforcments for earth