According to the following question:What is a loop device when mounting? A loop device is a file that acts as a block-based device. While I can understand that conceptually, how is this different from mounting any other file? For example, if I take a dd image of an ext2 filesystem and then mount it, I don't have to use the loop option- mount -t ext2 file.dd /mnt/mount-point succeeds. When should I use the loop option and what difference is there between mounting using the loop option and mounting without it?
When mounting, when should I use a loop device?
mount;loop device
If you don't have to use the loop option to mount a regular file, it is because mount is detecting this and activating it for you automatically.  You used to have to specify it manually.