ok whats the purpose of rooting
If you don't know, then I suggest you don't. You have to ask yourself, if there is anything you want to do on your phone that you cannot without rooting. If there is nothing, then I recommend you don't bother. An Android phone can already do so much without rooting. Just use your phone as is and enjoy it first.
Rooting does allow you to do more with your phone than you already can. There are some things that are locked down, partly for your safety and party because the carriers and phone manufacturers want you to use (or at least be aware of) certain services they provide.
My recommendation is to just use your phone unrooted. I'm sure you can already do many things that you want to do on your phone without needing to root. Don't get sucked into the ads because you may end up getting stuff that you don't really need. If there is something you want to do, but cannot unless you root the phone, then you should consider rooting it.
Personally, I used my phone unrooted for almost a year now. It wasn't until I started using both SwiftKey and Swype (both alternate keyboards) that I considered rooting my phone. You see, I use SwiftKey when the phone is in landscape and Swype in portrait. Normally, I would have to manually change the keyboard between SwiftKey and Swipe because an unrooted phone does not allow an automatic change of the input method without prompting the user. By rooting the phone, I can bypass this restriction and use an app that automatically switches the input method based on screen orientation. I did this last week. Other than this, there isn't any other reason I would need to root my phone.
He's on telus, so he'll have the I747M
The rooting method he'll need to follow is in the all things root AT&T sub forum
If you do decide to root your phone, use the rooting method for AT&T phones. This method works on Galaxy S3's for Telus, Bell and Rogers.