What is it used for?
It allows you to create preset data to be included in a table. For example, if you use your database to store a list of options to be used in a dropdown, you can seed (insert) these options when you (re)create that table on another target.
Excellent. While we are here. How do we sync the schemas across devices?
Just be careful with that video as there is a potential cause of confusion. In Georges demo there just happens to be two different connections shown, You do not use 2 different connections, only one.
The connections are per target so create one connection, set the development connection settings, switch to the production target and add the settings to THE SAME server connection