Sure: basically when large institutional investors go to sell shares they sell them in blocks. There are not a ton of people who can afford to buy these entire blocks, though they may want to buy some portion of them. HFTs add value by buying these large blocks, and then breaking them up and selling them in smaller chunks. This is called "market making." This added liquidity makes it far easier for both the institutional investor to sell their shares as well as for the final buyer to purchase them. So the value they provide is increased liquidity to the market.

Edit: Whoops, I'm incorrect. This is a description of market making in general, but not how HFTs do it (which have a variety of strategies). But the general premise is that the value provided is still increased liquidity. That being said, not all HFTs employ a market making strategy.