Map Contacts

If you're snorkelling and you do not have Map Contacts enabled, aircraft will never appear on the map screen. The first sign of them will be taking damage when you are hit by rockets or depth charges. You cannot solely depend on your mast-mounted radar detectors since a number of airborne attackers are not equipped with radar.

It's not that I don't welcome using it, it's just that wanted to know the reason as to why the game would consider map contacts as realistic

Realistically, somebody (not the Captain) was always manning a periscope while the boat snorkelled to look for visual threats and contacts. In SH3 you would have to do that in real time, for yourself for the time it takes to charge batteries or transit from A to B. So using Map Contacts On in late war patrols substitues for your periscope watch. It might offend some people's sense of "realism" but it is certainly a reasonable solution given the restrictions imposed by the "game".