Just to help clarify for everyone, fastboot is used in rooting. Actually it's required to unlock the bootloader and flash a custom recovery. If you boot into your bootloader, you will see fastboot as the first option. When connected to the pc it changes to fastboot usb when you're in fastboot mode. And you need adb and fastboot on your pc as you use both when rooting.
Remember commands like fastboot flash unlocktoken and fastboot flash recovery?
Now, htc also named one of their features fastboot... Why? Who knows, it just causes confusion. The fastboot that you uncheck in the menu is basically a hibernation mode for the phone. It prevents the phone from turning off fully, so that it can start up faster, or boot faster.
Some people prefer flashing roms through fastboot as opposed to from the phone. It's actually a great way for s-on users to flash roms with kernels as you can fastboot boot recovery into the phones memory and be able to flash the rom and kernel. Now we have 4ext with smart flash that allows this to happen for s-on users.
See we have everyone arguing over what fastboot is because it's confusing. It's frustrating that htc named their hibernation mode fastboot... Anyway, everyone was right about their points they were arguing, but the other side was right as well. There are two completely different fastboots on the phone. One used for rooting, one not.
I hope that helps clear things up. I'm not always the best at explaining things and i'm typing this on tappa so it's hard to keep up with my thoughts.
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