The cost of carrying value is useful, check here, because it lets you find out what the sentiment of the market is. When the value of the CoC is low then this means that the underlying asset value is falling in the market and vice versa.
The traders use the value of the CoC to understand what the sentiment of the market is. A major fall in the value of the CoC is an indication of the fall in the value of the underlying asset. So suppose if you see that the index CoC value falls by a huge amount then this will be followed by an index correction. When the value of the CoC rises for a stock future then this means that the traders are willing to part with more money to hold that stock. You would see the value of the stock go up.
The cost of carrying value could also be negative in value. When the futures price is at a discount to the price of the underlying then the cost of carrying value will be negative. This could happen for two major reasons. The first reason is when the traders are executing a strategy which is known as reverse arbitrage where they buy the spot and sell the futures. The other reason could be that the stock may be expected to pay a dividend. A negative cost of carrying value indicates that the sentiment is bearish.
You could use the value of the CoC to understand the bearishness or the bullishness in the underlying security. You use the CoC value along with the open interest value to know what the sentiment of the index or the stock is. The open interest is an indication of the number of open positions in a contract. When the open interest is rising and there is an increase in the CoC then this indicates that the sentiment is bullish. When the CoC value falls and the open interest is rising then this indicates bearishness in the market.
A fall in the open interest and a rise in the CoC value indicates the closing of the short positions. When the open interest and the cost of carrying both fall in value then this indicates that the long positions are being closed. The change in the CoC value when the contract is about to expire is watched closely by the analysts. If the cost of carrying value is high and a number of positions are being rolled over then this indicates that the market is bullish.