And a little espionage as well. Not to mention that, even though they are physically human, they are well trained enough to handle superhuman threats. And they've got plenty of street smarts to boot, which can really come in handy in certain situations.
Fun fact: There was an Avengers story where Earth's mightiest were battling the Elder known as the Grandmaster, who had seemingly gained control of death and sought to remake the universe. It was ultimately Hawkeye who beat the Grandmaster by tricking the him into losing a game of chance, thanks to a little sleight of hand on Hawkeye's part.