If you have an ace up your sleeve in a combat, you don't pull it out immediately. You wait until you can get the most bang for the buck with it. Something that will distract your foe is best used when it will allow you to seize a decisive advantage.

Or when it might possibly be your only way to avoid being killed.

Transcript

Darth Vader: And now, young Starkiller, you will die.
Darth Vader: I will have justice for my dead son!
C-3PO: You're doomed, Corey.
Luke: Maybe...
Luke: I was saving this to get Vader distracted so I could land a mortal blow, but screw it...
Luke: Obi-Wan never told you what happened to your son.
Darth Vader: He told me enough. He told me you killed him.
Luke: No. I am your son.
Darth Vader: No. No. That's not true. That's impossible!
Luke: Search your feelings, you know it to be true!
Darth Vader: No! Noooooo...!