They are just sketches to see what would look best. Da Vinci (along with all the great painters and artists) did a lot of these before he would actually sit down and produce something. Back then, they did a lot of research and sketching to get it just how they liked it. And that's basically what these are for. Just different variations. I've always thought the eyes were the best lol.
I'll eventually get around to scanning in full pictures that I've drawn from these sketches and some anime pictures I did wayy back in the day lol