Because it strips individuality from the Empire and gives nearly all of it's people the same grim visage. Everywhere you look, that's not "Sam" or "Bob", that is the Empire. The Stormtrooper helmet along with the Star Destroyer are like the symbols of Imperial force and power. As for Stormtroopers being being useless, I find that to be a myth outside of Endor.
Looking just at episode 4, the evidence suggests that imperials are very precise. Even Obiwan acknowledges this. They are precise to the point of subtly, carefully prodding and penning their target where they are instructed to let them go, all without fatally injuring them if so ordered. This is evidenced from Tarkin enabling the Falcon to escape; He did not want them dead. He was frustrated by the lack of leads to the rebel base and decided to let leia lead them right to their back yard. This is a common imperial tactic if Season 2 of Rebels is any indication, when Tarkin and Vader used the same tactic with great success to locate the Rebel fleet. So clearly it's not just all incompetence, at any rate. Sometimes they're trying to miss.
Also, Leia is an idiot.