Simply put, the GPL says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. By distributing the complete source code for GNU Octave under the terms of the GPL, we guarantee that you and all other users will have the freedom to redistribute and change Octave.
Releasing the source code for Octave has another benefit as well. By having access to all of the source code for a mathematical system like Octave, you have the ability to see exactly how each and every computation is performed. There are no black boxes that hide the details of any calculation.
Although enhancements to Octave that are written as function files in Octave's scripting language are not required to be redistributed under the terms of the GPL, we encourage you to release your enhancements to Octave under the same terms for the benefit of all users. We also encourage you to get involved with the Octave project and submit your changes for inclusion in future versions of Octave.