The purpose of marking to market is to keep futures settlement secure even when prices move. In Japan, clearing participants and the central counterparty, Japan Securities Clearing Corporation (JSCC), revalue open positions at the settlement price every day and exchange the difference the next day. Securities companies and their customers also mark positions to market every day. Besides listed derivatives, JSCC marks open CDS, interest rate swap, and over-the-counter JGB transactions to the latest prices at least once a day.
Mark-to-market is also used outside futures as a way of revaluing exposure. Regulation W in the US requires a member bank to have a policy for monitoring and controlling the credit exposure arising from derivative transactions with affiliates. The policy includes managing the exposure through mark-to-market requirements, together with credit limits and collateral requirements.
Consider a hypothetical example in which you buy one futures contract at 1,000, and each 1-point price move gives a profit or loss of 10,000 yen per contract. If the settlement price that day is 995, the 5-point fall times 10,000 yen gives a loss of 50,000 yen for the day.