F6 works great in TTL/Matrix with SB-28. In fact when my new F6 arrived in the mail a while back, it was dark and my first exposures were with the flash.
iTTL lets the F6 expose with flash up to 1/8000. In those cases, probably there is plenty of ambient light too, so the image can be over exposed (flash exposure plus ambient exposure) so, according to Nikon, the iTTL can 'balance' the exposure.
To do that 'balance' the flash needs to pre-fire. This, of course, will confound one's ability to use optical slaves. So the next 'feature' of iTTL is the requirement for radio triggering of remote flashes.
iTTL lets the F6 expose with flash up to 1/8000. In those cases, probably there is plenty of ambient light too, so the image can be over exposed (flash exposure plus ambient exposure) so, according to Nikon, the iTTL can 'balance' the exposure.
To do that 'balance' the flash needs to pre-fire. This, of course, will confound one's ability to use optical slaves. So the next 'feature' of iTTL is the requirement for radio triggering of remote flashes.