MacRumors notes that Facebook, which acquired the private messaging service in 2014, says they will use the information to provide better friend suggestions and targeted ads and offers to users of its own service.
By coordinating more with Facebook, we’ll be able to do things like track basic metrics about how often people use our services and better fight spam on WhatsApp. And by connecting your phone number with Facebook’s systems, Facebook can offer better friend suggestions and show you more relevant ads if you have an account with them. For example, you might see an ad from a company you already work with, rather than one from someone you’ve never heard of.
If existing WhatsApp users don’t wish to allow the app to share their account information with Facebook, they can follow the steps laid out on the WhatsApp website.