What they do is add volume to your intake plenum. This has been shown to aide in cylinder filling at lower RPM, smooth out throttle response and lengthen the effective wave pulse of the intake charge.
Most OE manufacturers tune their intakes to certain RPM ranges. Therefore changing the volume of the intake would slightly alter the target operating range of the engine. How much it changes the range depends on the volume of the spacer and the power band of the engine.
I am currently making a 25.4mm (1") spacer to see if it will help smooth out the throttle response on my 65mm throttle body.
Are they worth $80.... no. Do they add any real Hp..... no!