do you know the root directory of your computer?
with out access of the root directory.. you can not get into the directories of the system files and other important files. You are limited to the directory/ folders / files that they give you access too.
so carriers.. intentionally block the user access to the root directory on your android phone.
rooting... kills the block and gives the user full access to all files on your phone. good or bad.. it is for you to change .. update.. modify...replace.. and **** it up royally.
rooting does not change anything.. only kills the block that the carriers put in.