To require turret to have target in fov is a method to prevent turret targeting logic from getting into loops that occur when target exists near the fov edge.
The other option 'fov edge checks' also deals with the similar issue however in this case when turret is targeting a ship it actually targets very specific spot on the ship (or on a subsystem as the case might be) however if the game is unable to get valid FOV to the predicted impact position (ie. the point where the shot from the turret would intercept the intended aiming point on the target's hull) it may revert to targeting the target ship centerpoint. Now if you have ships side by side this event will cause turrets to jam and only occasionally start firing at visible targets and then cease again. Even though there would be valid visible spots for the turret to target. Now with the flag the turret tries to find a position on the edge of the FOV as close as possible to the intended position and aim (and shoot) at that direction. There are no guarantees that it would actually hit the target. But its better to at least try to shoot at the target than for the turret to lockup and cease functioning altogether.