why do I have to explain what we are going to use the business account for

Account purpose is part of customer due diligence, not credit assessment

The bank asks about the purpose of the account because it is required to record this under the customer due diligence obligations of the Money Laundering and Terrorist Financing (Prevention) Act (Wwft). That due diligence does not concern whether your business is in sound financial health — that is a different question, with a different source, as you can recognize in the question an insurer asks about financial health. The purpose of the account serves to verify whether the expected use logically aligns with the nature of your business: a webshop sending large amounts to a country with no connection to its activities deviates from the expected pattern, and the bank must be able to explain that deviation in its own file.

This falls into the same category of questions as source of funds and UBO structure: the bank builds a client profile, and the account purpose is the part that records the expected transaction pattern. Supporting documents are usually not required here — unlike the source of fundswhere a financial statement or contract is often requested. A clear, factual description of business activities is usually sufficient: "purchase and sale of construction materials to contractors in the Netherlands and Belgium" is verifiable, "miscellaneous trading activities" is not. If you are uncertain whether an answer from your chain file fits here, that is precisely the moment to answer "no" instead of guessing.

Basis: customer due diligence under the Wwft

The obligation to conduct this due diligence follows from the Money Laundering and Terrorist Financing (Prevention) Act, which requires banks to understand the business relationship before and during its establishment — including the purpose and intended nature of that relationship. This is separate from information already publicly recorded under the Trade Register Act 2007, such as the business classification code (SBI) of your business; the bank asks about the account purpose precisely because that public register does not show how the money will actually flow.