extras.ubuntu.com Why is it needed?

Question

linux

Good afternoon, hackers!

When installing Ubuntu asks a lot of questions, among which there is a choice of additional repositories.
With the main ones, everything is clear - free / closed, supported or not.
But it was not possible to find sane information from the original source about the partner and extras repositories.
Why are they needed and what do they contain?

Thanks in advance for your replies

Answer 1

Regarding the partner, it is written in the repositories file:
This software is not part of Ubuntu, but is offered by Canonical and the respective vendors as a service to Ubuntu users.
About extras - too:
This software is not part of Ubuntu, but is offered by third-party developers who want to ship their latest software.

Answer 2

partner is skype, adobe flash, mp3
extras are add-ons for unity, etc.