2) Currency futures markets exist to allow you to bet that the value of the currency will decrease, by offering to sell you the currency at price below the current market rate in the future.
In markets where there's no easy/legal way to bet that prices will decrease, asset bubbles are common, as "bulls" bid up the price, and "bears" just sit on the outside waiting for the bubble to pop.
If bears could bet that the value would decrease, this would lower the price of the asset today. In this way, price signaling can allow the market to reach an optimal shared opinion about the "true value" of an asset.
There are a lot of cryptocurrency bears who think that the long-term value of Bitcoin and Ether are approximately zero, that cryptocurrencies are in an asset bubble, and that this is wasting valuable time and money that the economy should spend on other things. By betting against cryptocurrency, bears could help to end the bubble in a safer and slower way.
(Or so they say. Sometimes future markets can exacerbate price instability when the whole market behaves irrationally.)
(The other reason futures markets exist is to allow people holding the currency to reduce ("hedge") the risk that the value of the currency will decrease. Thus they can make small bets against the currency, and if it rises, they'll still make money, and if the value falls, well, at least they'll win their bet.)