I have been at several lunches with executives type events at different places. The groups are small so it makes more casual back and forth questions much easier, which is useful.

From my perspective, the real value of the lunches was strictly building rapport across the company. At some places high level employees (doesn't necessarily have to be management) often feel unapproachable and they feel more abstract concepts than people. Having lunch with them re-enforces they are also people, people you may even get along with.

Other companies I've been at only do Q&A sessions with large groups of employees, and this is still useful, but the format doesn't help the culture. The answers you get are often the company line, and you cannot dig deeper. The people answering the questions are still abstract identities that appear to tout that company line, with no real personality.