Instant Videos has several purposes. First, nowadays, video is one of the main focuses of Facebook. This feature will increase the number of video viewers and provide a better user experience.
Also, downloading a video when a user connects to WiFi lets them not have to use data packets and can encourage users to watch more videos on their phones.
In addition, it could be, like Instant Articles feature that allows news to be displayed quickly on a Facebook page.
Instant Videos works for Facebook to gain more control over its video ecosystem and bring benefits to content creators who meet their rules. However, at this time, it seems Facebook has not done that yet.