A man-in-the-middle attack is a sneaky way to order some enemy to do something that benefits you. If you can intercept a communications channel and relay messages between a sender and the intended recipient, modifying the message on the way through, then you can gain the appearance of authority for your modified message.
Players can use this as a trick to confound the enemy. Or GMs can have the villains use it to set up some sort of adventure scenario. Imagine if the Joker got his hands on the Bat-Signal, or could ring the Bat-Phone, what sort of mischief he could get up to.
[Reminder: Our guest commentators have not seen Rogue One. Part of the fun is seeing how their untainted impressions re-interpret the movie through the lens of our comic.]
Yes R2, for the good of the mission, and the good of the party, fire at empty space. It's not like people's immediate response would be Chinook like, "How did we miss?". Maybe the targeting sensors are miscalibrated. That would be checked. Or maybe someone is lying to them. Their incoming data streams hacked.
Or... wait, "party"? R2 doesn't know these strangers. Why should R2 want to help them?
No, what Pete should really be thinking about is not "How to destroy this super weapon", not "How to destroy this super planet". Pete should be thinking "How can I control this weapon as my own?"
Which... is kinda like "I can fire at will".
But we don't have Will in this party.
Slightly more serious: In 1733, I mentioned that blowing up at least part of the compromised security center made sense (the antenna, the area where the intruders are, the entire tower in the worst case.) In 1736, R2 says he can control the weapons and do what makes military sense for the other guy.
... did I say more serious?
So, how much of the transmission does the Peace Moon know of having happened? If they think it's been transmitted, they want to fire on the fleet. If they think it is still being transmitted, they want to fire on the tower. Heck, the Peace Moon has Tàrkin and Vader. Maybe they just want to plug a leak, and plan on firing on the planet anyways. Maybe they've had one too many reports of incompetence from the planet.
Either way, they're going to fire into someone else's Charlie Foxtrot. And we know what that means...
C-3PO: You're saying you can take over the Peace Moon?
R2-D2: No need.
R2-D2: It's obviously about to fire at our fleet. I've just isolated the communications channel of the targeting system.
R2-D2: I think I can inject an alternative target.
Cassian: So, either we sacrifice ourselves or... the Empire destroys the Rebel fleet, stops us stealing the plans, and keeps their data centre?
Bria: And Krennic would rest in peace!
C-3PO: There's another option. Artoo could redirect the beam into space. The fleet could get away with the plans and you'd still have a chance to get off the planet.
GM: Yeah, that would probably work.
R2-D2: You want me to fire the biggest weapon in the Galaxy at... nothing??