They are closely linked. In fact, the markets move in lockstep. How else could they be used to hedge?
You can't directly arbitrage with inter-exchange transfers, but traders can still arbitrage the difference between futures vs. spot, which can significantly affect the market. This is possible because the delta of a futures contract is not equal to 1. When fomo is strong in the futures market, traders pay thousands above the spot price, betting that when delta=1 that the price will be that much higher.
Bitmex and Okcoin have long had an effect on the spot market. Often times, it's useful to watch for massive liquidations on the futures markets to determine when the market has hit extremes and will likely correct.