even if the movie fantasy of being able to steal and field-strip an attacker's gun is highly unlikely for 99% of gun varieties, knowing what a shooter needs to do in order to use his gun is useful for knowing how to prevent them from doing so, how to make a gun (relatively) safe.
If the worst of the worst of the worst happens, knowing how to fire said gun may be useful, but more important than that IMO is knowing how to *stop* said gun from firing and endangering anyone else.