Importance of open interest
Open interest is different from trading volume. Volume represents the total buy and sell in the market. But in OI, when one trader with 10 option contracts sells those contracts to a new trader, it may increase the volume trader but the total contract does not change the total open interest.
Thus increase in OI describes the flow of market trend. OI also acts as both support and resistance. When there is high OI in 10500 CE and 10400 PE, chances are the nifty closes at 10450 to cancel out both OI buildup.