The only benefit of unlocking the bootloader, is that the phone can then be rooted at will without any repercussions. The bootloader can also be re-locked at will, though that kinda defeats the purpose.

Benefits of rooting? Well, this gives the ability to customize things beyond what the maker (and google) might normally permit.

Eg. Better Ad-Blocking. UI customizations. Titanium Backup (the best backup/restore app on Android), etc..