Facebook is stepping up efforts to get people's phone numbers. It's not part of its usual quest to gather up as much info on people as possible, at least not on the face of it, the site wants your phone number in case your account or even Facebook itself is compromised, it's a security feature.
Unfortunately, it's a needed security feature. Passwords alone are no longer enough to keep people secure. On the one hand, many people use the same password everywhere so once it's compromised attackers can use it on dozens of sites.
The flipside of this is people who use so many passwords that they tend to forget them. Granted, most people tend to visit Facebook often enough, but plenty still use the "forgot password" button quite frequently.
With your phone number on record, Facebook can send you a message with a new temporary password in case there are problems so the site is notifying users, all of them, that adding this info could be useful.