The reason why you are asked to verify your Facebook Business Manager account is that the integration of WhatsApp that works by scanning the QR Code is not an official integration with WhatsApp.
In fact, this integration is based on the Web version of WhatsApp and not on the official WhatsApp Business API. For this reason, if one day WhatsApp were to completely change the functioning of WhatsApp Web, our integration (even if the probability is minimal) could stop working.
For this, we ask you to verify your Facebook Business Manager so that you can possibly migrate your account to the WhatsApp Business API at any time (you will be required to use a new number) in order for us to be able to guarantee the continuity of our service.
In addition, we always want to clarify: if WhatsApp is a fundamental channel for your business, we recommend that you opt for integration with the WhatsApp Business APIs in order to guarantee you a stable and reliable service over the long run.