The Divi Builder Plugin is a great way to take the power of the Divi Builder and add it to any other WordPress theme.

Note: You do not need to have the Divi Builder Plugin installed if you are already using the Divi theme or Extra Theme. The Divi Builder is built into the Divi theme & Extra theme and the plugin is not needed.

This is also mentioned in the plugin download page in the ET members area:

Check out this article that will help you to download and install Divi Builder plugin:

https://help.elegantthemes.com/en/articles/2165950-how-to-download-and-install-your-divi-builder-plugin