Why do people recommend rooting a device before flashing a custom ROM? If rooting isn't required to flash a new ROM (unlocking your bootloader is), why do many people recommend rooting your device before flashing it?
I think this one really depends on the device and procedures involved in installing a new ROM. For Nexus devices, there's really no point because you can just fastboot oem unlock the bootloader and install your new recovery with fastboot flash recovery [file].

Other devices, however, may force you to use tricks that require root in order to unlock the bootloader. For example, in order to install a ROM on a device which doesn't support fastboot, you may need root in order to install a custom recovery.