Without a grocery store, you would have to drive around to multiple farms to source all the food you want to buy, burning gas all the way and when you get there theres 5 other people fighting over the last egg and bidding it up to silly prices.
HFT matches up buy and sell orders efficiently, and the people who run these operations take a cut. If they take too much, someone else will undercut them in a race to the bottom (which is good for regular customers)