From The Economist:
ENGINES on airliners are highly efficient when they are in flight, but not when operating on the ground. When a plane is taxiing under its own power, the engines burn vast amounts of fuel. A Boeing 747 can consume a tonne of fuel and emit several tonnes of carbon dioxide during an average 17-minute taxi to take-off. And when the aircraft lands there is likely to be another long drive to the passenger gate. Which is why there are various methods being developed for aircraft to use other means of propulsion while moving around an airport.
If you think about it, this was an area ripe for innovation for a long time now. Obviously having a second engine for taxi purposes on board is ruled out for weight and space reasons. Something external is obvious since these negatives are shed when it’s detached. Since engines would only need to start shortly before takeoff it would also mean less noise at airports.
It’s interesting that in engineering you focus on the primary use case and make it efficient. Once you’re done you continue to refine and make it more efficient. The folly is when you forget about other low hanging fruit near the edge cases, in this case when the plane is on the ground. While not a huge savings, it can potentially add up.