I think they allow you to make more errors on the line and still pull the holeshot. . . Or still have a good jump. When I was actually racing hard every weekend all those years before 2005 I think that launch device really helps.
On concrete starts it doesn't help much. I found that you just have to sit with your weight over the pegs and creep off the line then get on it hard once you're in the dirt. Except when you're at Pleasure Valley, the button is needed because they have the gnarliest rough broom finish a concrete start has ever seen, you can light it up off that start without burning a few moto's off the tire.
Helps the small guy. But I'm sure Ricky Carmichael would need a holeshot launch jack because of that belly.