So factorys can be set to attack a target or to attack move, sending their creations to do the command once built. Used this to great effect in GW by having bot factory's suppress the enemy's base with fodder while I set up inferno facto.....I mean vehicle factory's to finish the sucker off.
Not only that, but they can self-assist with Fabbers, so you can have a factory devoted to outputting large numbers of Fabbers.
And here we have a wild noob, in his natural habitat. He is defending his territory with a large force of - wait. We must zoom in on this one factory that is draining all of his eco as fifteen unused fabbers continue to self-replicate. This unusual strat is usually labeled the "Nanobot" strat, because in only a few hours, this noob will have covered the entire planet with fabricators, ensuring his victory. How clever.
I recently found out this also works wonders for defense, set your factories to a wide area attack of your own base, and every unit built will auto-attack any enemy unit near your base, which is actually way more effective than me doing it myself I'm not very good After attacking they will just stand there like a lemon though. Worked quite well versus the AI's assaults, I do expect a human opponent to be able to take advantage of it, still better than paying attention