Hedge your existing positions
An investor with a collection of different shares might short an index to protect themselves from losses in their portfolio. If the market enters a downturn and their shares start to lose value, the short position on the index will increase in value – offsetting the losses from the stocks. However, if the stocks increased in value, the short index position would offset a proportion of the profits which had been made.
Alternatively, if you had a current short position on several individual stocks which feature on an index, you can hedge against the risk of any price increases with a long position on that index. If the index rises, your index position will earn a profit, counteracting a proportion of the losses on your short stock positions.