Processing banks require that the currency merchants charge in their stores match the settlement currency, i.e. the currency of their bank account.
This means that if you bank account is in USD, you must have your website charge customers in USD.
Some merchants would like to sell to customers all over the world, but do not want to set up True Multi Currency
The most common solution for this is to allow customers to take on the conversion costs through their bank i.e. In-Cart Multi Currency
If our site sells in USD and a customer goes through checkout with a EUR or CAD credit card, the site will allow them to get through the checkout process in USD
Their bank will the make the conversion for them and charge them a fee.
There are some Disadvantages to this solution
This solution makes the products more expensive for the customer due to the conversion costs.
It also obfuscates the true cost, as the customer does not necessarily know the cost of the product, conversion rate or bank fees.
There are also some advantages
It is much simpler for a merchant.
The merchant does not need multiple websites, processing banks or bank accounts.