Duty of Disclosure
When you complete an insurance proposal form, the insurer does not make an attempt to verify everything you write down. It would simply be impractical and too costly to do so. Equally, the insurer may have no way of knowing if you tell lies or just 'forget' to disclose a material fact.
As a safeguard, insurers ask that you tell them anything else that may affect their assessment of your insurance application. In other words, they are putting a 'catch all' clause in their policies. They are saying 'tell us anything else we need to know'.
In some respects this clause is controversial and can lead to arguments in the event of a claim. Nevertheless, insurance is an imperfect science and for the system to be workable, the insurers claim such a clause is necessary.