Different skills

Freeform self defence teaches you how to deal with an assailant.
Non-cooperative self defence teaches you how to deal with an assailant who fights back.
Pao chui teaches you how to deal with someone who can match your skills and counter your counter.

San da teaches you how to deal with unpredictability. It challenges your ability to adapt, change and improvise.
You are free to mix and match a wide variety of skills against an opponent who is seeking to do the same to you.