The idea behind it I think is that the watch that would have been used wouldn't have been welded to the periscope and could have been carried around with you. Having it while on the bridge is useful for determining if a torpedo has hit or missed while perhaps manoevering. Also while on the NAV map you can track a 3.15 plotting measurement while drawing your lines and whatnot. Overall it is very useful to me.
__________________
|