Because you are under-utilizing a resource. If I have a 1024x1024 texture and half of it is unused, the unused bit will still be put into graphics memory but not used. It is like setting aside space for nothing, but, in games as with any piece of software, the goal is not to bloat the program (least space used is better for loading times and other stuff), so detail that could have been put into memory isn't. That wasted space has the potential to become useful.