Every procedure I found to root my phone required to unlock the bootloader as a prerequisite. Well, I have an idea of what a bootloader is and it seems that there is something that prevents it from being changed, which is implied in the answer to this question. Also, the answer to this question says that it is not necessary.
The question remains: Why are rooting instructions asking to unlock the bootloader?
Aren't these separate things? Or do some rooting procedures also change the boot loader?