Site clearing has many purposes but the main purpose is to remove any shrubbery, trees, stumps, rocks, boulders, man-made structures, or any other material that is currently on the land that needs to be removed so that the environment can be groomed to be built upon.   If building on any plot of land is going to take place, it must be groomed and prepped so that there is a strong foundation.  Just like anything else, foundations must be strong enough to withstand the structure being built on it.