The phone number is too important to them for tracking purposes. It allows them to tie your instagram usage to your facebook account to your whatsapp conversat

The phone number is too important to them for tracking purposes. It allows them to tie your instagram usage to your facebook account to your whatsapp conversations even if none of them are linked together, and then create a big map of all of the people you interact with on the different platforms. The more phone number information they have, the more powerful their map.