Swap arises due to the overnight interest rates for each currency being different. Since currencies are always traded in pairs, you always need to borrow one currency in order to buy another, so it follows that you have to pay interest on the loan, but you also receive interest on the currency you are holding. Also, I think, broker charge swap as one source of income to the company. If no swap charge traders tend to hold positions for the longer term. However, since most of the brokers provide swap-free accounts recently, most of the traders also tend to choose swap free account.

Brokers charge swap, also known as overnight financing or rollover fees, to compensate for the costs associated with maintaining leveraged positions overnight in the forex or other markets. When traders hold positions overnight, brokers essentially lend them the funds necessary to maintain those positions. Swap rates are determined by the interest rate differentials between the currencies being traded. If the trader's position involves borrowing a currency with a higher interest rate and lending a currency with a lower interest rate, the trader will pay the swap fee. Conversely, if the position involves borrowing a currency with a lower interest rate and lending a currency with a higher interest rate, the trader may receive a swap credit. Brokers typically charge swap fees as a percentage or fixed amount based on the notional value of the position and the prevailing interest rates. These fees ensure that brokers are compensated for the cost of providing leverage and maintaining positions beyond the trading session.