1. bigger injectors supply more fuel to your engine to allow you to make more horsepower.. if you build up the engine and dont give it fuel, it will starve and say bye bye engine.
2.the reason their soo expensive is simple.. people need them to make big power
they are expensive becasue they are rather complicated things. think of the cost this way: if you were building up a car with a carb, you would spend roughly 5-600 for a really good carb(demon, holley). now compare the cost of a bigger carb with the cost of injectors, id say the injectors are a deal.