Why buy External Audio mixer?
 

Whats the use of external audio mixer in non-linear editing suite? Do I really need one. I shoot fully digital. In what situation I need to use external audio mixer?

If you work with analog sources an audio mixer will be more useful. i.e. VHS, microphone for recording VO.

The mixer can help simplify your wiring and set levels.

It sounds like you don't need one.

2- For some audio applications, you can get control surfaces to control the mixer in the computer software. But that's something else and does something different.