Photo via Sam Azgor/Flickr (CC-BY)
In a blog post, WhatsApp detailed the change, noting that coordinating with Facebook means the messaging service will be able to do things like better track metrics about how people use it and fight spam messages.
As for what Facebook wants with user infomration from WhatsApp, well, it's never been one to turn down a data point. Having access to a person's WhatsApp number will allow Facebook to offer better friend suggestions—presumably based on contacts—and better target ads to users.
One might expect it to be modeled off Facebook Messenger, which now allows users to interact with customer service representatives (often bots) and reportedly may one day let companies directly contact users to deliver offers and advertisements.
WhatsApp noted that privacy was still its priority, and end-to-end encryption ensures that no one other then the sender and recipient are able to see the content of the messages.
Still, some may view it as a step back from the stance company co-founder Jan Koum, who promised users after WhatsApp was bought by Facebook that, "Respect for your privacy is coded into our DNA, and we built WhatsApp around the goal of knowing as little about you as possible."
How to opt out
For the time being, users who find the change too invasive can still opt out of it through one of two methods.
H/T The Verge