It's also to test your listening skills. Many optional objectives can be learned by simply paying attention to any special requests that automatically come in via Codec when the mission starts or as new developments happen in the middle of a mission.
That sounds a lot like the mission in GZ where you have to retrieve a tape from the command tower. You can finish the mission successfully by retrieving the tape and extracting, but if you pay attention to what happens and Miller's reactions, it's evident the first tape was only a trap and your informant had sold you out. There's another tape to be found, but the game never explicitly tells you this.
(Which is awesome)