What does unlocking the bootloader have to do with rooting?
Unlocking boot loader is NOT mandatory for all devices. Unlocking, is to facilitate flashing of unsigned images as in custom ROMs. Even if you unlock bootloader, for rooting only, it exposes you to security concerns (see question linked at end of this answer ).
If your bootloader is unlocked already, there is no need to unlock . There are devices with unlocked bootloader and for those you can proceed to root directly. For example
Majority of devices seem to require unlocking
This question explains well things about unlocking
Is it possible to unlock the bootloader without root access?
Rooting procedures do NOT change the bootloader. That would be dangerous


Also see , Why does unlocking the bootloader wipe phone?