HEre's a bit from what I read in a few history books-
In Traditional/feudal Japan, although the Samuari did have loads of swordfights in battle,etc. they did their best to keep blood of f of themselves or touch dead bodies. To do so caused some sort of difilement, and the person in question would go to a shinto shrine to be purifed.
One of the ways you could show your true skill and bravery in a large battle was to take the head of a enemy general or leader. If you could do so, it was a HUGE amount of honor on you and your sept in the clan-it was even bigger if the head you took was one of your Daimyo's (clan lord) bitterest enemys.
soo..the Tanto blade was handy for such a thing. It allowed a way to remove it (assuming your Katana did not do the job) or carry it without getting the blood on yourself or touching a dead body. Also carrying the head of a General around a battlefield was a BIT de-moralizing to the enemy....
Then of course, you would take said head to your clan lord for viewing, and then after the viewing was done, you would send the head back to the enemy as a slap to the face.
As someone said, the Tanto was the utlity knife for the swordsman..
ETA-oh yeah..in 1600 at the battle of Sekigahara, Lord Tokugawa took 40,000 heads in the battle that established the Shogunte that ruled from 1603-the late 1800s(meji restoration). Now how is that for kicking ass and taking names?