What was Nintendo's intention of adding the Warp Zones into the original Super Mario Brothers? Was it meant to be an easter egg for players to find? Was it used by the programmers while debugging?
Generally when debugging a game under development, a programmer puts in various ways to skip ahead to later levels (such as key combos) that don't always make it into the final released product. This means warp zones were probably not for testing purposes.
I suspect they were just fun and interesting secrets.
Historically, most Mario games have some way to bypass large sections of the game (Warp Zones in SMB1, Warp Whistles in SMB3, Star World in SMW) as well as powerups that allow you to skip individual levels (raccoon tail, cape). Miyamoto's philosophy is that you shouldn't get stuck trying to get past a level if you don't want to.