Absolute file offsets demand a particular memory layout or some extra bookkeeping that could be avoided with relative offsets. If I were to write a JPEG parser, I could write a function to handle one particular segment and not have to worry about other segments because relative offsets makes parsing them independent, compared to TIFF where I need to maintain a directory of things and make sure the offsets land in the right place.

I think parsing file format with absolute offsets is similar to handling a programming language with all GOTOs, compared to relative offsets which are more like structured control flow.