The QB calls an audible when he believes changing the play will lead to a better outcome than running the huddle play.

There are a huge number of defensive formations and offensive plays built to exploit each ones weakness, so if changing a play can allow taking advantage of a better matchup than the called play, many coach's systems allow the QB to make a read and change the play.

The other stuff is there to communicate various sub play options, like how a blitzing player should be blocked, and then there's a ton of chatter designed to prevent the defense from predicting the snap count.