Yeah, as Coolasbeans says when open water swimming being able to breath on either side is important in case of waves (tide or wind generated) coming in and preventing easy breathing. Also useful for sighting - again, single side breathing you might be unlucky and just have sea to look at.
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Better on neck, quicker than breathing every other stroke, every 3 strokes, symmetrical allows for roll so all in all should be faster.
Might be wrong
Or you may have someone swimming close to you... You don't want an elbow in the face
Gives you something else to look at whilst swimming