A locked or unlocked bootloader is what gives you access to “root.” “Root” is another big word in the Android community. ... If you “root” a device, it means you have “super user” access or “administrator” access to the operating system that runs on your phone.

Is unlocking bootloader necessary for rooting?

No, it's not necessary to unlock bootloader for rooting. But flashing custom ROM or recovery is not possible without unlocking the bootloader because locked bootloader verify the ROM or recovery being flashed for company's official signature and custom ROM doesn't contain the official signature.

What does unlocking the bootloader do?

Unlocking the Bootloader allows you to install custom firmware on your Android phone and gives you full access privileges to make modifications to the phone. Such modifications could involve changing some of the pre-loaded software, or could even include replacing the entire operating system.