why does the bank want to know how much cash flows through our company

Cash is an indicator in customer due diligence

The bank asks this because cash is more difficult to trace than a bank transaction, and that makes it a standard point of attention in the customer due diligence that banks must conduct under the Anti-Money Laundering and Counter-Terrorist Financing Act. The bank must assess the risk of each business relationship, and a high proportion of cash in turnover increases that risk profile, regardless of whether there is anything wrong. This is therefore not a suspicion, but a standard question in the risk profile that the bank must maintain for every customer.

Concretely, the bank usually wants to know what part of turnover comes in as cash, from which activity that comes (for example a shop till or service charges paid in cash) and whether that proportion has changed compared to the previous year. If you don't have those figures precisely, an estimate with a clear explanation of the source of that estimate is usually workable; a mere denial without substantiation is not. The same bank often also asks questions that are related to this risk profile, such as what the origin is of the company's assets and at which banks the enterprise maintains accounts — cash is for the bank one piece of a larger picture, not a standalone question.

Where this comes from: the customer due diligence of the Wwft

The obligation to collect this type of risk indicators follows from the customer due diligence that the Anti-Money Laundering and Counter-Terrorist Financing Act imposes on banks: the bank must not only know who the customer is, but also how money flows through the company, in order to be able to identify an unusual transaction. This is a different perspective than, for example, an insurance company, which looks more at financial health than at cash flows — which is why the bank's questionnaire comes to a different conclusion than that of the insurance company, even though the subject seems the same.