Why do you need to root your Android phone and what does it really do?
In simple explanation rooting is a way to provide you a higher privileges to operate on your device. Just like in Windows operating system where you have an option to run a program as an administrator. Rooting gives you super power to have full access to your device.
Without rooting you would not be able to alter the operating system files. Once your phone is rooted you could change the system themes, core applications, boot images and recovery images.
In additions you could run special program that require more control over the system or uninstall factory default pre-installed apps, you could easily backup your system file, backup downloaded app to the SD card and many more.
Some people does not want to root their phone because by doing so they would lose the ability to accept OTA updates or they could not get support from their service provider.