Sometimes only parts of the binary file are relevant for a particular use-case: for example a program reading and writing the EXIF data of a RAW file probably does not want to decode the Bayer sensor data. Or you have a huge container file with a directory at the end of the file and you just want to display the contents of the directory. In that case seeking would be much faster than reading and parsing. And sometimes writing is not necessary at all: for example a program, which sorts camera RAW files into directories according to the time stamps of the EXIF data. And sometimes you do not know how to decode parts of the file, because the format is proprietary and only half of the format is reverse engineered.
But file-position works fine. I just tested it. Thanks!