The act of shorting a stock puts downward pressure on the stock price, similar to the way buying a stock puts upward pressure on it.

Suppose you see an ongoing pump-n-dump---sketchy pseudoinformation being passed around to raise the price of a stock someone has already bought, so they can sell later. If you short the stock, you act to reduce the price excursion and potentially help the eventual victims.

Suppose it's not a pump-n-dump, but rather "irrational exuberance"---people buying a stock and raising the share price for non-economic reasons. Short selling applies alternate pressure on the stock price, reducing the effects of a subsequent correction.