Some examples are:
Hedging: Miners, who has purchased mining machines/cloud mining power/mining tokens, could lock in mining returns by shorting Hash Rate Futures (longing mining difficulty).
Arbitrage: When the cost of mining is lower than the price of Hash Rate Futures, an arbitrager could buy mining power and short Hash Rate Futures to profit from the difference.
Speculation: If the difficulty implied by futures price is lower or higher than traders' view, they can trade the Hash Rate Futures with leverage for profit.
For simplicity, below three examples ignore the transaction fees or edge cases where mining could be interrupted due to accidents.