why do I have to tell the new insurer which policies we already have

Existing policies are part of the risk assessment

An insurer asks this to determine whether your business is already insured elsewhere for the same risk, and to see what the overall risk profile looks like. This is often called "overlap" in the industry: two policies that would cover the same damage. Insurers want to rule that out, and they also want to know whether you have been insured for longer or have recently switched — that says something about your claims history and how you deal with risks. So it is not a control question about you as a customer, but part of the acceptance: based on your answer, the insurer determines the terms and the premium.

In practice, it usually comes down to the question of whether a liability insurance for the business is already in place, and sometimes also other business policies such as legal protection, cyber or business damage. If you do not have those, you simply fill that in — an insurer does not expect every business to have everything already covered. If you are unsure whether something counts as "insurance" in the question, the note at the answer is the place to report that; the insurer will assess itself what that means for the acceptance.

No legal obligation, but an acceptance condition

This is not laid down in a law such as the Trade Register Act 2007, which only sets out which basic data of a business are registered. The question about existing policies comes from the insurer's own acceptance policy: it determines itself what information is needed to assess a risk and to be able to make an offer. So it is not a legal information obligation as with supervisory authorities, but a condition to be able to conclude or renew an insurance. If the answer is missing, the acceptance usually stalls, not a legal procedure.