Why does an aircraft use less fuel while landing as opposed to takeoff and climb?
Ground effect plays very little here. 

When taking off and climbing you need to add kinetic energy and potential energy to the plane in terms of speed and height. Getting that energy requires burning fuel.

When landing you want to get rid of that energy. Easiest way is to turn down the engine (or off when something breaks) and let drag slow you down and trade height for speed.

When taking off the plane will also be heavier because it will have more fuel on board. So it takes more energy to speed it up and get it up high.

It's very similar to climbing a hill and descending a hill, going up takes energy and going down means you can coast.