Futures were initially invented for hedging: imagine you own 1000 BTC and for any reason (liquidating, etc.) you don't want to make any P&L (profit and loss) on these => you sell futures contracts for 1000 BTC. What you gain on one side you lose on the other, so your P&L will be 0 (minus fees, you can think of these as a sort of insurance).

This is useful for instance for a fund that needs to neutralize its positions while closing them over a span of several days/weeks. If you expect a future payment in BTC and don't want to expose yourself to the risk that Bitcoin will crash: you short the equivalent amount in futures.

Because futures contracts are (usually) much more liquid that actual assets, they are very useful to short whenever you have the actual asset in your books and don't want to expose yourself to the risk of it losing value.

Not sure if I'm explaining all this very clearly...