They sure do, it means they can make one fork and its suitable for a lot more people/frames, to the customer it means you can play about a bit on the trail and go for them wound down for more agile turning, less wander on the climb's, or wound out for faster going down bits. A good example of a bike thats suited to adjustable forks is the On One 456 designed to work well with 4" 5" or you guessed it 6" travel forks.
If its a 100mm frame, I wouldnt stray more than 20-30mm either side of what the frame was designed for though, and personally dont have much interest in going under the designed size, only a bit longer.
oushdi boursh.