Oh, I dunno, it's an abstraction.
The idea is that you are not photographing one specific ship (although that's what you actually DO), but, rather, gather intel on the port.
Which, I think we can all agree, is important information. Amount of traffic at the port, types of ships docked there, defenses, etc - this is very important information for massive follow-up raids (such as carrier group air strikes or bomber raids).
It'd be more realistic if the mission orders allowed for requirements like 'loiter outside the port for a minimum of 14 days' or 'photograph ships entering and leaving the port at least at least twice a day for 7 days' or something like that.
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