A currency pair contract is based on spot contracts, which are equivalent to 2-day futures. Without rollover, you would need to close your current contract and open a new one every two days. To make trading seamless, banks handle this process for you and charge a swap fee to cover the interest costs of buying and selling the same currency for different value dates. Forex brokers pass this cost on to clients. Swap rates are updated daily and provided by our liquidity providers.