Reason for rooting #1
I have a (very) old HTC Touch HD (circa March 2009) that came with HTC WM 6.1. Pretty much boring user-interface, as all Android/IOS users would have said.
Now it is a custom ROM WM 6.5.3 (WEH), with an Android user-interface and the last OS update was early this month. I can easily customise the phone to more than these 5 Homescreens (see attached) and more.
Before this, the phone also an Android OS with a custom ROM from XDA.
If I do not root my phone, this could not have been done, and I would have needed to buy an Android phone (which I will try to avoid). My phone is working perfectly as I wanted it to, and yet gives me more than 5 days on a single charge (1350 mAh battery) unlike the "modern" power-guzzling Android phones.
Regardless of the original manufacturer of the phone, rooting a phone gives the owner the freedom to do what he wants with it (assuming he knows what he is doing) and not be locked-down by the manufacturer.
Furthermore, rooting my phone, flashing custom ROMs and updating radio ROMs, make the phone more efficient and productive. It also helps me to know more about my phone and how to make most of it.