I get questions like this from my kids all the time. Sometimes I can answer them, sometimes I can’t. So, any ideas on why humans have fingernails? My guess is for grasping things better, or they could be a remnant of a distant evolutionary cousin.
Ever trim your nails too short and try to open a soda can?
Have you not noticed that all mammals have “nails” of some sort? You know, claws, that sort of thing. There’s no real evolutionary pressure to get rid of them, so they stick around. Plus, they do have the occasional use, like peeling oranges or opening CDs.
Opening sunflower seeds.
I can open sunflower seeds with my tongue. 
(OK, I crack them with my teeth first.)
And fingernails are for scratching.
picking up dimes.
They seem to serve some purpose in primates, because all primates (lemurs, monkeys, apes) have them. We’re primates, so we have them, too. Maybe they helped our primate ancestors in grooming-- that’s a big part of social primates’ behavior repertoire. And the vast majority of primate species are social.
Yeah, I think this is one of the benefits of fingernails, is that they protect the finger tips.
If humans have fingers, why don’t fish have handgers?
I once had a fingernail pulled out with pliers (under local anaesthetic, but still bloody and unpleasant - not recommended).
Believe me, you can’t do much with a finger without a nail - it can’t grip or push down. Writing with a pencil, holding anything by the finger tips, hitting a keyboard - all next to impossible until the nail grows back.
Why do humans have fingernails?
They’re to nibble on during periods of extreme stress. 
Fingernails are useful in a variety of ways (as I’m sure you know) like scratching, pulling out splinters, picking up dimes 
, prying open food, grip, holding onto something very small, pulling out hair, scraping off dead skin like scabs or picking your nose or cleaning your ears, and do offer some protection to the back of the fingers. They’re also good for fixing ingrown toenails!
And those mutants way back when that were born without much of claws or fingernails couldn’t peel fruit or dig up food and starved maybe, or couldn’t groom their friends/mates and were ignored and not mated with, couldn’t scratch their own butts and killed themselves going crazy with itching… Evolution at work. Or something.
I always thought their main purpose was to stop your fingertips flapping about. Which would be gross and not very controllable.
Try picking your nose without fingernails.
They call 'em fingers, but I never seem 'em fing.
Oh. There they go.
Boogers.
Hemlock’s answer is in line with what I vaugely remember from physical anthropology in undergrad; fingernails evolved to increase tactile sensation and make it easier to digitally manipulate objects by evening out the pressure on the opposite side of the finger pad. I’ll try and dig up a proper cite when I get the chance later in the day.
Boooooo!!!
Guess what finger I’m typing with! 