1. To try to "persuade" the other guy to turn his key. If the other guy won't do it, even at the threat of being killed...then carrying out that threat won't help.
2. To prevent a saboteur from damaging LCC components. (You need two REASONABLY INTACT consoles to send the launch votes.)
3. After they've turned their keys, and they realize that they're about to be on the receiving end of about a megaton or so, they can take the final exit if they wish.