Privacy is always the main reason for using a VPN (Virtual Private Network), whether you use it for Facebook or anything else. It encrypts your traffic so that bad actors or snoopers can’t see what you’re doing on Facebook or even that you’re using the social media platform at all.
A VPN can also come in handy for people living in regions with oppressive government censorship. Many authoritarian regimes restrict or outright ban social media use, but by routing your traffic through VPN server in a different country, a VPN may help you overcome such restrictions.