Oh, that's easy, when you have multiple IBANs, you get an opportunity to make international transfers without paying any imposed fees. It's a super useful feature for those who admire travelling and can't sit at one place.
Hence, advise you to think on Easy money or easy green plan, as far as I know they have the maximum possible number of local IBANs.

IBAN stands for International bank account number, and this number is used when you are making transfers, whether you want to receive or send money.
You might not need any other IBAN than the one you use.
But if you often transfer funds, and the transfers are done in multiple currencies, then having an account with IBAN from the local currency could significantly reduce the costs of your transfer, as well as improve the speed of the transfer.
With multiple IBANS bunq is allowing you to localize transfer.
I hope I was clear enough