If you believe wikipedia, the extra point was once worth more than a touchdown. You scored a touchdown to get the opportunity to kick an extra point.
http://en.wikipedia.org/wiki/Convers...iron_football)
The try/convert is among the oldest parts of the game of gridiron football and dates to its rugby roots. In its earliest days, scoring a touchdown was not the primary objective but a means of getting a free kick at the goal (hence why the name "try," more commonly associated with rugby today, is still used in American football rule books), and thus early scoring rubrics for the game gave more points to the subsequent kick than the actual advancement of the ball into the end zone. The related term "conversion" is still used in both rugby union and rugby league to refer to extra points scored by kicking the ball through the posts after a try has been scored.
By the start of the 20th century, touchdowns had become more important and the roles of touchdown and kick were reversed. By this time the point value for the after-touchdown kick had reduced to its current one-point value while the touchdown was now worth five. (This later increased to six points in American football in 1912 and in Canadian football in 1956.)[citation needed]