provision of liquidity. If HFT didn't exist, you would still be able to buy and sell financial instruments, but it would be less convenient. Maybe you go to buy something and the offer doesn't appear until 10 minutes later. With HFT it's always available. Imagine your local supermarket. What do you need it for ? You could drive straight to distributor's warehouse and buy food directly from the distributor or the farmer who grew it. The local supermarket is a middleman which provides convenience of execution. Same with HFT.
Bad analogy. HFT is not like a supermarket. HFT would rather be the guys at the local supermarket running behind you and right in the moment you're about to grab something off the shelves they grab it buy it and put it back on the shelves so you can buy it for a higher price only. And they'd be allowed to do so because they own parts of the supermarket. Oh those guys don't exist at supermarkets? Well funny...
