Round LE makes the transfer of air from the bottom surface to the top surface smoother.
Is in that critical area where the acceleration of the upper air flow happens from zero (at the stagnation or separation point below the LE) to maximum speed (area of lowest pressure) on the forward portion of the upper surface of the wing.
This is more pronounced as the AOA increases, reaching a max just before stall.

A foamy with flat wings has perfectly square LE and still flies.
However, proper streamlined airfoils are more efficient regarding lift/drag.

Check this video out:

http://www.youtube.com/watch?v=6UlsArvbTeo