To get lucky
He had a captive roll the dice and then use the item that morphs into a random card, so that he can get good cards from it (though you can only get up to class A cards with the dice using this method). Since he could blow the captive if they don't agree it's not like they have a choice. He doesn't roll them himself since he doesn't know what the misfortune might be if he rolled the unlucky side.
The 1/20 Chance to get unlucky with the dice is always a certain death. So the Bomber Team punished others in rolling the dice for him to get lucky and win his missing Rank A and B Cards. That way they didt have the Risk of dying because of the dice.
You can't die from items in GI, but getting unlucky would probably put you on the brink of death and certainly isn't something you want to find out one way or another.
So this Guy got killed by the Mashine because it exploded after the Dice went on the unlucky side.
They also fear the bad luck roll because they dont want to die.
The dice is presented this way through the Manga.