When using the forward renderer with MSAA it will use Alpha to Coverage for masked materials which provides (if I recall correctly) 3 steps of translucency. The main benefit of this is that you can use the extra steps of translucency to antialias foliage and other masked materials.
Ben Golus did a great write-up of the feature and how to use it for antialiasing: Anti-aliased Alpha Test: The Esoteric Alpha To Coverage | by Ben Golus | Medium