Leverage.
So I had a look at live data for an example. As of writing, 1GBP = 1.28 USD. Let's look at some example scenarios, assuming I believe 1 GBP will be worth 1.41 USD (+10%) in 3 months and want to make as much money as I can with a $100k investment.
- I buy USD from a regular bank or currency exchanger and wait 3 months. If the price goes up 10%, I make $10k. If the price goes down 10%, I lose $10k. If nothing happens, I lose nothing.
- I buy GBPUSD futures contracts. I need $7346.25 margin to open the contracts and $5877 to maintain them (per https://www.interactivebrokers.com/en/index.php?f=marginnew&...) and I control 62500GBP ($80k) per contract. With $100k, I can buy 13 contracts and control $1.04M. If the price goes up 10%, I make $100k. If it goes down 10%, I lose $100k.
And then, for fun:
- I buy 86 options for a GBPUSD futures contract at a strike of $1.28 for $99k. If the price goes up 10%, I make $593k. If the price is at ~$1.30, I lose nothing. If the price is below $1.28, I lose $99k.
I'm no expert on futures however so there might be something wrong here, though the answer is definitely leverage.
We can trade GBP/USD between each other with as much leverage as we like without any future time conditions. All we need do is agree to pay each other the difference in value as the price changes, multiplied by our agreed leverage.
At some point, one or other of us can close the deal and settle the outstanding amount.
If you can't strike a deal directly, you go to an exchange or clearinghouse of some kind that will introduce buyers and sellers. Once again, leverage can be created without any forward or future.
Forwards came first. You would go to an investment bank tell them you wanted to hedge whatever you had and they would write up the contract between you and them. Obviously there is no secondary market for these things and two contracts could have different terms in them.
So futures were invented. They standardized the underlying product (e.g. a type of wheat, a particular duration on bonds, a fineness of metal, etc), the expiration date, the delivery location (or financially settled), and the size. Now since all these contracts are fungible a market can be formed to trade them back and forth.
Those are basically the only differences.
2. Holding spot currency position is not "zero storage cost", you need to pay daily interest - futures has this built-in in their price.
3. Futures are leveraged
4. Futures contracts are standardized - which makes finding counterparty much easier and allow market makers.
5. Futures traded on regulated exchanges (CFTC in US), unlike spot FX - which is more similar to how crypto exchanges operate.
What's the point in splitting hairs. A future is a standardised form of a forward contract, generally so they can be traded more easily. It doesn't affect the underlying financial concepts.
Do the calendar spreads, implieds, volatility, high performance matching engine, and market makers.
Existing exchanges have been amazed to find out that these 'hacks' aren't even the end of the money train, you can create derivates of the hacked money to let customers re-deposit and trade even further! It is an amazing new world of finance!
Additionally futures aren't just for "betting" they are also for offloading risk. For example a company might expect an sale in another currency and might want to "convert" that earlier to offload the risk of making another purchase in a different currency.
Instead, I could have just bought the 0.79 GBP immediately and then waited 3 months, and then I would have profited even more: 0.79 - 0.7 = 0.09 GBP.
What does the existence of the futures market allow me to do that I can't already do in this scenario?
My point was that in cases where production is cyclical or storage fees are nonzero, it might be difficult/impossible to trade on the spot market, or it might be prohibitively expensive to hold the item yourself. But currencies don't have either of these problems.