With Unity's new 2D tools, a very common thing people will be attempting is to make platformers, and an extremely fundamental part of platforming are platforms which can only be interacted with from the top.
In 3D, this would be fairly easily achieveable, either by using a one way mesh or by placing an empty GameObject on each edge of the object in question (or using the scale) to find the lowest point of the object, and determining whether that point is higher or lower than the platform they're trying to board. At that point, you could simply use Physics.IgnoreCollision to refuse collisions between two.
However, this function does not exist in 2D, and we don't have one way edge colliders. There are cheap kludges like using triggers to remove velocity, but obviously at high velocities this will result in objects burying themselves in your platforms, and strange physics once things fall on top of each other.
This function would also be useful for a plethora of events, like players colliding with platforms/enemies but not each other, and bullets not harming players. For now layers can be used, but this is a cheap fix.
So if you would implement a Physics2D.IgnoreCollision function, this would resolve a lot of important issues for platformers.