It lets you shift once with the front and once (or quickly) with the rear to maintain the same cadence.

With typical overlap, I'd only need to shift the rear three or four cogs at the most to keep mostly the same cadence when I'm changing chainrings. It's really simple when going from the small ring to the big -- I can just swing both shift levers the same direction, shifting the rear three cogs bigger with one big sweep. Going from big ring to small, I just tap the upshift lever three times and I'm good.

Having twenty distinct gears with no overlap means that I'd have to shift the back several more times whenever I wanted to switch chainrings.