That's what meetings are for, right? Or even just the ability to walk to someone's office/cube/desk, and talk to them about important things. You can establish a culture of communication within a team that doesn't intrude on personal time.
There is a lot of use in say, complaining that a way you are going about a problem will work, but seems too damn hard. Say for example, I see a 4 hour solution to a problem that should really be solved in 30 minutes. Often coworkers who know some systems better, or have a different way of viewing the problem will see a much easier way to solve it by leveraging tools I don't know about or approaches I didn't think of.
Certainly, these informal discussions over lunch aren't strictly necessary, and certainly, having some alternate way of making these discussions happen is very useful, but they are just often helpful.
It's also not the case that having lunch with coworkers is by any means mandatory, or that taking the lunch for yourself is frowned upon.
