What is the benefit of low efficiency speakers?

I know 8 ohm speakers are easier to drive than 4 ohm speakers, and most decent amps can drive nearly anything, but why are some speakers made with loads that drop down to two or one ohms?

Are they designed to cause, or allow some certain aspect of the sound to come through that would not otherwise be heard? What's the point of making a very difficult to drive speaker? Did that sentence make sense?