Continuing the analogy: Some trading is as if there's a person standing between the two people trying to make a deal and can hear the offers before each party can react and then make their own offers to each party to capture some of the difference (re: HFT profit).
Explanation: In some forms of HFT the key advantage is that they can buy the market data faster than other people so they can see upcoming trades before other participants (undoubtedly there are better citations but here's a decent NYT article): http://dealbook.nytimes.com/2010/06/11/opening-up-the-market...
Of course, if you're not desperate on selling right now and or the risk for the trader is too high (i.e. the minimum price you'll sell right now is higher than what the trader is willing to offer you right now), nothing happens.
Concluding, traders do add utility to a market. And as others have already said, with high freq trading it mostly results in massively reduces spreads (and the temporal utility as outlined in the laptop example essentially disappears).