why do most file formats define a start of file marker? [closed]

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For instance jpeg (JFIF) has a SOI (start of image) marker. One can argue that it can be used to identify the type of file, but I'm looking for more sound reason with supporting examples.

Answer 1

These are often referred to as "signature bytes" and their primary purpose is simply to aide in validating the file. Some file types contain additional signature bytes elsewhere in the file (ie: BMP format), and some contain none at all. The latter kind still generally provide some other means to validate the file using a variety of techniques, such as checksums, stored file size and the like.