Good question! And that is why the governments and central bänks around the globe watch and (try to) regulate these markets.
Their prime benefit to the world in general is that they help to provide liquidity to the real money markets.
Whenever a company buys or sells things in foreign currencies, or borrows money in another currency, or an investment fund or pension fund wants to invest in something denominated in another currency, or anyone buys or sells securities,shares, bonds etc denominated in another currency, or you buy something on E-Bay or change money for your holiday abroad, real currency transactions take place.
These transactions are mostly carried out via bänks who function as market-makers. They constantly quote a bid and offer with a spread in between and are prepared to buy or sell according to the customer's wish and the actual funds are transferred to and from the customer's bank accounts in the two currencies. So the bank has assumed the other side of the transaction. The bank now needs to decide whether to hold the position or enter an opposite position with another market-maker - and so the money goes round.
But the efficiency of this market requires constant and deep levels of liquidity so that prices move smoothly up and down and not jump all over the place and with periods with no price available anywhere, For this reason speculative trading is allowed.
Therefore these pseudo markets or financial derivitives fulfill this role of providing liquidity by allowing one set of customers to secure a desired price level by contracting with another opposite set of customers willing to take the risk of the opposite position. There is a financial market, and there is a petrol station and there is a concert, but we do not need to be the ones actually using them on order to trade them provided the prices of the real market are mirrored in the derivitives by normal free market activities.
Read a little about the futures markets for commodities like oil, metals, coffee, grains, etc. These are huge markets used by producers, consumers and speculators. The futures contract is an obligation for a specified unit of a product at a specified quality at a specified price for a specified date in the future. These contracts are freely bought and sold without any physical delivery until the actual expiry date. Then, if any contracts are still open, they can be swapped into a physical delivery. But in practice, at least with financial futures, they are closed out before the expiry date with just a P/L settlement. Our market is similar but without the set expiry date.