Because the cost-of-carry model ensures that the return on a fully margined position in a futures contract mimics the return on an underlying spot deliverable, futures contract returns are often used as a surrogate for cash market performance.
Is this English? If yes, could someone try to rephrase it? Thanks 
If you take a long position in a commodity futures contract and post 100% margin, you get the same result as you would have gotten had you simply bought the commodity itself.
As above. The forward curve in the futures market reflects the cost of carry so that the arbitrage between spot and futures markets is not possible. If this weren’t true you, could make a riskless profit.
Looked simple when you explained it. Thanks gents!