How do we really benefit from carrying over futures in profit from one month to next as we need to square off the current month futures and buy the next month at the existing price

The benefit is not outright, I mean by rolling over your position that is closing this month contract and opening next month will not put money on table upfront.Also to roll over you need to close existing position and enter into next month contract at a premium.So,one will be benefited only if the contract moves in their direction covering the premium paid to enter into new contract.