- ROOT - WHAT AND WHY

1. WHAT IS ROOT AND  ROOTING 
ROOTING is simply geekspeak to describe the process with which the user gets administrative access to the phone. You bought the phone, but you are not it's administrator! Wierd.
Historically speaking, as Android is an offshoot of the Unix Operating System, the security measures of the latter are applicable. So, since in Unix, only the "superuser" really could do everything with the system, the same applied to Android. And "su" is the command used to get superuser access. ROOT is another term for superuser, which is more commonly seen tagged along with other terms rather than solo, like "root access", "root folder", "root directory", "root permissions" etc.

2. WHY ROOT
The Android system is not just the software that runs the phone, it also includes a load of stuff gratuitously bundled by Google and the phone company. Like Youtube. Which I don't need. Now, Google could have allowed users to uninstall Youtube, just like any other application. But NO. So, you need to ROOT. Other reasons to ROOT include the need to MOD the phone by loading other flavours of Android or just to try out stuff that others said was not worth doing or, better, should not be done.
Another reason to ROOT is if you want to reinstall apps without having to waste bandwidth downloading the app afresh everytime. I don't know why Google doesn't allow keeping a offline copy of an app for later reinstallation. To the best of my knowledge, there is no way to take a backup of an installed app for later restoration, without  having ROOT permissions.

On a more serious note,  once ROOTED, you get the right to customise your phone to the hilt. BUT with RIGHT comes RESPONSIBILITY. Since it is easy to uninstall stuff that could be critical to the Android system, doing so will leave your phone unusable. So once you are ROOT, YOU are RESPONSIBLE for all consequences (not the phone company), in other words, ROOTING voids WARRANTY. Now you know why ROOT is not freely handed over.