You don't have to root a device to install anything, you just have to enable "Unknown sources" (Android will prompt you to enable it when you try to install an APK). A non-negligible amount of people do this -- it's how the Amazon Appstore works on everything (save for the Kindle Fire): http://www.amazon.com/gp/feature.html?ie=UTF8&ref=mas_gs...
to install anything useful to your phone you have to root it. e.g. try installing tethering or VPN apps when your operator image denies it.
also, all processes that involve rooting/flashing will involve one step that you remove the firmware locks from the device, allowing a thief to peek at your data at will even if the phone is password protected.
*downvoters, also contribute to the discussion saying how you enable tethering on, say, a thunderbolt bought from at&t. thank you.
Someone has already covered tethering, so I'll just emphasize there are plenty of other useful things you can get by enabling "Unknown sources" without rooting: other app stores (Amazon [noted upthread], GetJar, MiKandi, ...) often with their own exclusives, software under development (Swype Beta, Mozilla's Aurora and Nightly, ...) and so on.
Separately, if you managed to buy a Thunderbolt from AT&T you have much bigger problems than getting tethering working. The Thunderbolt is a Verizon-specific CDMA/LTE phone that doesn't support any of AT&T's networks (AT&T's LTE is on a different band).
yeah, the thunderbolt wasn't mine :) had no idea which operator locked it. but it was locked beyond hope.
and i wasn't saying 'non-google-store-sources-aka-unkown-sources is a bad thing. i was just saying it's not enough to 'do what you want' with your device.
