Why invest in shorts?

You can find a more detailed but still very simple explanation here, but really basic initial reasons are:

1) It is a good way to make money

2) It is a good way to hedge your longs, similar to "hedging your bets". If you have some long and some short you make it possible to make sure you make money either way the stock goes.

Both have their risks but if you do your research you can have both shorts and longs and end up with more money in the end.