Although it's not nearly as important on a vertical installation as it is on a floor, you still want to make sure you get full coverage in order to bond the tile well. The problem with large format tiles is that they tend not to be completely flat, and as a result, with a normal sized trowel (like a 1/4x3/8), you could very easily have a majority of the tile not even touching the thinset. That's the reason for using the larger notch.