iTunes let users add functionality of 'Remember Position' to mp3 files.
This is VERY useful for AUDIOBOOKS so you do NOT have to remember the position of the last playback position.
Imagine if the file is 1 hours long and you're currently on 0:22:00 minutes.
WITHOUT that functionality, the file is RESET EVERY TIME BACK TO 0:00 AT THE BEGINNING!
YOU CAN ONLY ADD THAT FUNCTIONALITY IN ITUNES.
IF YOU USE ANY OTHER ITUNES OTHER THAN THE ONE ON THE MAIN PC, THE IPHONE IS RESET (WIPED CLEAN!)!