Your Facebook account is valuable for fraudsters as if they can get access to it, they can get access to all your 'friends'.
Most of us login using an e-mail address and password, and as we often use the same e-mail address and password in multiple places giving it away is a problem elsewhere, not just on Facebook.
And if you have an e-mail address at Hotmail, Gmail etc. then the fraudsters know where to go to try and get at your e-mails - especially if you use the same password there as you do for Facebook - and they can then cause havoc with changing your details all over the place as they will receive the confirmation messages and authorise them via your e-mail account.
A common scam is the Messenger video asking if it's you:
If you click on it, you'll see a screen like this that impersonates the Facebook login screen, although note the URL in this example is 'agilecrm.com' and not Facebook:
If you enter your e-mail address and password, you've just given the fraudsters access to your account (and potentially your e-mail, your bank accounts, PayPal, Amazon, eBay...). It's as easy as that.