First, Low is a great a capturing likeness and mood. He does this by focusing on the eyes on the face. They don't call them the windows to the soul for nothing. The eyes also can establish a system to measure by.
The third reason also involves gestural lines. Low's wild line work allows for the artist to make discoveries. These discoveries lead one down a visual path to success. When you find what works you darken the lines to emphasize out what is working. This is like singing that amazing note your voice hits a little louder so your audience can really appreciate you got it right.