As Steffani implies, at work, I can use the word "lunch" to mean either the midday break time or a meal or both. "To lunch", IMO, implies taking the break with the possibility of having the meal while "for lunch" implies having the meal. To take the break for the reason of a meal, perhaps?
In other words, if I say "I'm going to lunch now", you should not assume that I've eaten when I get back, but if I say "I'm going for lunch", you know that food is my purpose.