You need the lens to correct abberations from the angle difference the light path follows to your eye. At an angle, the light path is longer than what's exactly normal, or straight on. Hence, the fringe is "in the wrong place", or appears so. I guess that if you practice enough with a transmission flat, the experience can be aquired without resorting to a lens every time you check a flat.
This becomes a nasty aspect if you check fringes for straightness through the back of a concave test reference. It's extremely unreliable in this particular situation.
Mark