Lunch is somewhat important for humans. Eating. Exchanging ideas. Talking about senseless stuff. Talking a bit about work. Getting to know each other. Lunch.
Especially in IT there is a huge amount of projects failing - not because of the coding capabilities - but because of social problems.
Lunch may come to the rescue. Or at least bringing team members closer together. But if you have to force people to eat lunch together something is broken.
[1] Joel writes about it: http://www.joelonsoftware.com/items/2011/04/28.html
[2] Those two chemists explain it, too (Ben and Brian): http://www.google.com/events/io/2011/sessions/programming-well-with-others-social-skills-for-geeks.html