The standard answer is liquidity, which is a really abstract concept, and one I still struggle to conceptualize (how hft supposedly creates liquidity is still too abstract for me, for instance).<p>Here is one recent example that was eye-opening to me: <a href="http:&#x2F;&#x2F;www.bloombergview.com&#x2F;articles&#x2F;2015-07-07&#x2F;can-you-really-game-index-funds-" rel="nofollow">http:&#x2F;&#x2F;www.bloombergview.com&#x2F;articles&#x2F;2015-07-07&#x2F;can-you-rea...</a>.