loop-device

Why does one need a loop device at all?

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I previously used to create image files using dd, set up a filesystem on them using mkfsand mount them to access them as mounted partitions. Later on, I have seen on the internet that many examples use losetup beforehand to make a loop device entry under /dev, and then mount it. I could not tell why one would practically need an image file to behave as a loop device and have its own /dev entry while the same behaviour can be obtained without all the hassle.

When mounting, when should I use a loop device?

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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.