Because you want to kill things and take their stuff somewhere besides an empty 10x10 room. Generates a grid of any size filled with colored areas indicating difficult, impassable, damaging, elevated/lowered or special terrain.
Adding the flavor is all up to you - is that square of damaging terrain lava, or a pit trap? That depends on if your party is in a volcano or an ancient tomb. But don't worry - there's examples of all the types of terrain included, in case you have trouble getting inspired.