Sure, Facebook is paying their users $20 per month to install this VPN (plus referral fees to encourage others to install it), but according to one security expert:
If Facebook makes full use of the level of access they are given by asking users to install the Certificate, they will have the ability to continuously collect the following types of data: private messages in social media apps, chats from in instant messaging apps – including photos/videos sent to others, emails, web searches, web browsing activity, and even ongoing location information by tapping into the feeds of any location tracking apps you may have installed.
You may recall that Facebook had its Onavo Protect app banned from Apple's App Store this past August and which was practically the same in effect, but Facebook has been separately offering this Project Atlas VPN through offers at Amazon since 2016, mostly to users in the age range of 13 to 35.