Teams usually do not want to expose their starters to grunt work on special teams. It falls to reserve players. That's why you'll always hear in preseason even though a backup might be playing well, his path to the roster depends on his ability to contribute on special teams. Barring a ton of injuries, it is the only action fringe players get.
Some players like Larry Izzo make a career solely out of special teams play. Contributing here can even turn disappointing draftees like Justin Miller into sought commodities on the open market. It can also extend careers, leaving the door open for something greater in the future. Take the case of Wallace Wright.
Wright has provided the Jets with terrific work on coverage teams for three years. This is the sole reason he has stuck. Now an opportunity is there at wide receiver, and Wright has reportedly impressed at practices. This opening was not there in 2006, 2007, or 2008. Had Wright not been such a valuable contributor on special teams, he would not be on the roster today or have a chance to contribute at his natural position. Special teams work kept him in the game.