The Marker reacts to 512 levels of tilt, which lets you draw and write with a paper-like feeling. By tilting the marker, you can add shading and vary the thickness of your stroke. The tilt is only available for certain sketching tools. Read more about it here.
The Marker is also pressure sensitive (2,048 levels to be exact) which allows you to draw thicker and thinner lines just by adjusting your pressure on the marker.