Why do we need to set loop devices when mounting ddrescue images?

In various tutorials I see the following command:

mount -o ro,loop,offset=123 harddrive.img /mnt/loop

but from reading the documentation I can't understand what are loop devices. Why would harddrive.img require different options than /dev/sba? From my own testing mount -o ro,offset=123 harddrive.img /mnt/loop seems to work fine so what is the point and the meaning of this option?