In theory, that's what they do. In practice they are credited with utterly destroying several companies that might have been able to survive otherwise. No one knows for sure if they would have made it, but they had absolutely no chance to survive with short sellers in the mix. The stock market often encourages a herd mentality which isn't necessarily based on reality (e.g. the dot com bubble of the mid-90's). Short sellers have a way of sending that herd mentality into overdrive.