The Intermediary Bank details that we make available along with your international multi-currency account are extra information that you can use if you are going to receive payments in USD.
In SWIFT transfers, payments do not go directly from the sender bank to the beneficiary bank, they go through one or more intermediary banks.
Therefore, when you or your payer is filling out a payment form, in addition to asking for the beneficiary's details, there may be a field to inform an intermediary bank as well. That's where you can use that extra information!
If there is no intermediary bank field on the form, just leave it without that information.