This one kept me awake last night: It can’t be so you can pick your nose, then you would only need one.
I though you typed with your penis. Shows what I know.
Nah, when I type with my penis, it tends to look like this:
6tt6ub 66n56hv yyv6g57 ct7586
'cause it’s too big to peck at one key, you see…
smaug
That’s a little more what I was looking for, although the other answers have certainly been entertaining. A cite would be great if you can find one. (I’m trying to teach my young’uns not to believe everything one reads on the 'Net)
refusal -
This is very close to what the great L.M. Boyd said. “Fingernails keep your fingertips from sliding around on the bone.” Or words to that effect.
That’s very interesting. The upshot is that fingernails also work as a support for the front of the finger?
What exactly happens when it’s missing? Does the skin roll around to the back, or does the tip bend back or what?
I always thought fingernails were just for scraping small bugs off the skin when they bite. A support function would give better reason for the toenails to still be here, too. 
-k
I used to say that humans have lips to keep their mouths from fraying, but now I don’t say much at all.