Let's say that your data is hosted on a database server accessible only from inside your office network. Normally, you'd have to be onsite or using a VPN to work with the data. With an extract, you can take the data with you and work offline.

An extract file contains data extracted from the source. When you save a workbook, you may save it as a Tableau workbook (.twb) file or a Tableau Packaged Workbook (.twbx) file. A workbook (.twb) contains definitions for all the connections, fields, visualizations, and dashboards, but does not contain any data or external files, such as images. When you save a packaged workbook (.twbx), any extracts and external files are packaged together in a single file with the workbook.

A packaged workbook using extracts can be opened with Tableau Desktop, Tableau Reader, and published to Tableau Public or Tableau Online.

There are a couple of security considerations to keep in mind when using an extract:

• The extract is made using a single set of credentials. Any security layers that limit which data can be accessed according to the credentials used will not be effective after the extract is created. An extract does not require a username or password. All data in an extract can be read by anyone.
• Any data for visible (non-hidden) fields contained in an extract file (.hyper or .tde), or an extract contained in a packaged workbook (.twbx), can be accessed even if the data is not shown in the visualization. Be very careful to limit access to extracts or packaged workbooks containing sensitive or proprietary data.

The story is told of an employee who sent a packaged workbook containing HR data to others in the company. Even though none of the dashboards displayed sensitive data, the extract contained it. It wasn't long before everyone in the company knew everyone else's salary and the original individual was no longer an employee.