The speech was for the audience.

Hola Garry

In this sentence that you're mentioning: 

El discurso fue para la audiencia.

(The speech was for the audience.)

we're using the verb ser as explained in the lesson, since we're talking here about "the recipient of an action", the same way we'd say "las flores son para mi madre", for example.

This is why we're using "fue" and not "estuvo", as the latter is the preterite of "estar" and it not appropriate for this meaning of "being for someone" that someone being the recipient. 

I hope this clarified it.

Saludos