The advantage of rooting a phone is it gives you access to the entire operating system of the phone so that you can change it to suit your needs.
Think of your Android Tablet like a Windows PC.. Some people do not like Internet Explorer. So they install Mozilla Firefox along side it and are happy to do so.
Some power hungry users do not even want Internet Explorer on their computers as it takes up space and resources as they do not use it. Removing this from Windows is a nightmare and you need specific privileges to be able to do it.
The same situation exists for Android users, I for example do not use Google hangouts, G++, Exchange Email, Facebook. I also want to underclock my phone for increased battery life as I do not play games on it.
I want the functionality of a command like SSH terminal and other applications, I do not like the default launcher, I want the very latest OS upgrades and I do not want to wait for them. I want my phone to back itself up and all its data nightly to my cloud account..... Rooting enables you to have full control of your device and ability to do all these things and so much more.
Get it wrong and you can end up with a non functioning device known as bricked