You might need the insurance policy number on the following occasions
You need the policy number when you go for the policy renewal or update. It may be needed to verify your previous / current policy. Furthermore, you can check if you can get a no claims discount or not.
When you submit a claim request, you need a policy number to claim your damages from the insurance company.
If you have lost your insurance policy document, you may need your policy number to give to the insurance company to issue a duplicate copy of the insurance policy document.