Ion engines are extremely efficient propulsors in a vacuum (they have a high Isp), and therefore useful for applications such as deep space probes, satellite stationkeeping, and interplanetary travel in general! One reason they are efficient is that the energy used is supplied to the engine in the form of electricity. This means you can use solar power or even nuclear batteries to provide power, which is much more efficient in terms of mass and space than carrying fuel. The only consumable you need to carry for thrust is the ion particle supply, usually xenon or some other inert gas. You can conserve fuel by using a more powerful electric field in the engine; a particle tossed out the back twice as fast yields twice as much thrust for the same amount of particle supply.