Now, it could be nothing, but I came to a personal realization just how valuable a telephone number could be for such a well-positioned company to do a deep-dive into a person's background just by using their mobile number as the key to tie additional records to the user's identity. Or their activities before, during, and after a company's conference.
To answer your question, I believe that Facebook wants the (mobile) telephone number of their users to facilitate any number of additional uses, and "account security" is one actual use that lets it slide under the radar of most people. Clever! (Or depending on your POV on privacy and personal security, unfortunate!)