why does the bank want to know who my customers are

The customer of your customer is a risk assessment for the bank

The bank is not asking this out of curiosity about your turnover, but to determine how risky the flow of money through your business is. This is part of the customer due diligence that banks must conduct under the Anti-Money Laundering and Counter-Terrorist Financing Act. A bank must not only know who you are, but also understand what kind of transactions flow through your account and with whom. If you supply to one large, well-known buyer, that is a different risk profile than if you supply to hundreds of small, changing parties in different countries. The bank translates that into a risk classification of your account, not into a judgment of your customers themselves.

In practice, the bank usually asks for a general picture: the type of customer (businesses, individuals, government), the sectors, and sometimes the countries where your sales come from. Rarely does the bank want a list of individual customer names — that also does not fit well with what they are allowed to do with that information. If you are unsure whether a specific customer name must be shared, that is a moment to look at what is being asked exactly and whether an aggregated answer (industry, region, number) suffices. This subject also touches on the question at which banks the enterprise maintains accounts, because banks sometimes place those answers next to each other to get a consistent picture.

Where this is based on: customer due diligence under the Wwft

The obligation to conduct this due diligence is set out in the Anti-Money Laundering and Counter-Terrorist Financing Act, which requires banks to continuously monitor the nature of the relationship and the transactions of their clients. This is separate from the business register, where only the fixed business data are registered according to the Trade Register Act 2007 — that source says nothing about your customer base. If you want to see how this type of question relates to what an insurer asks, look at why the bank asks different questions than the insurer, because the legal basis and the purpose differ significantly there.