The Engine spends more time in the correct rev band and is more fuel-efficient. Your kick-down on acceleration is smoother too.
Better acceleration as you are closer to the power band.
e.g. You can accelerate from 18mph to 155mph in 6th gear in my car. However it's much faster if I use the other gears on the way through. It will cruise up hills in 6th, but it is more efficient to use a lower gear in this scenario too.