Ditto what V and SK have said.

I coach people to ride the Death Ride in the Sierras. 130 miles, 15,000 feet of climb at elevation (up to 9000 feet). Sure, not everyone needs a granny gear. But a lot do, and those who do don't spin like mad or fall over or anything. They just spin up the hill. (And save their knees).

If they were racing a course with one or two high gradient "walls", but rollers otherwise, and if they were all as young and skinny as my cyclocross racer boys, then they wouldn't need it. But for the the average endurance cyclist tacking a long endurance pace and length mountain climb a granny gear is anything but overkill.