Just a few months ago, it was a free-for-all on Facebook when it came to videos. They were just getting off the ground and gave us all of the options at our fingertips, including a “call to action” button that we could add to the end of our videos.
That has since been taken away, but now it has arose from the dead once again, but this time on paid boosts. Facebook doesn’t like to send folks away from their website unless they are making money off of it, so now the call to action button has become a benefit of paying and boosting your posts. But once your boost ends, the call to action button disappears, so nothing permanent like the way they had it before.
One workaround is that you can always add the call to action link at the end of the post about the video, but the button is always preferred. They know this and have now monetized on it.