Because futures and options are traded in Chicago and equities are traded in NYC (well north jersey) there are opportunities to trade equities and their derivative counterparts. For instance SPY (S&P 500 ETF) in NYC and ES (S&P 500 futures contract) can be traded against each other as they move. However the guy who gets their first will get most or all of the profit. So you need servers on both sides and the lowest latency possible links between them. For instance a microwave network is being built which shaves a couple of milliseconds off the existing special fiber cable that traders have been using.