"For Mirrodin!" is a keyword on certain Equipment that creates a creature token when the Equipment enters the battlefield, then immediately attaches the Equipment to that token for free. You get a 2/2 red Rebel creature token (in the original Phyrexia: All Will Be One cards), and the Equipment comes pre-attached, so you're getting a creature plus an equipped weapon in one card.
The nuance people miss: the token-creation and attachment happen as a triggered ability when the Equipment enters, not as part of casting. This means it can be countered or fizzle if there's no legal creature to attach to (though the token is always created). Also, the Equipment itself isn't a creature—only the token is—so removal that hits creatures won't kill the Equipment.
In Commander, these are efficient because they dodge the usual "Equipment is a do-nothing card without a creature" problem, providing immediate board presence and pressure even into empty boards.