When you create an Extract, Tableau copies your database into a Hyper file (since Tableau 10.5) on your computer. Then, the Data Source is no longer linked to the database but to the Hyper file.
The first advantage of the Extract is that it's optimized for Tableau, meaning that no matter the speed of your initial connection, you will have excellent performance. Keep that in mind. If you think that your Dashboard is slow, the first thing to check is whether you are using an Extract. You are also able to work offline because the Extract is a local copy of your database.
Unlike a Live connection, you don't observe instantly the changes in the database. To see the changes, you need to refresh the Extract. When you update it, Tableau ...
