The reason to cancel meetings and seminar visits is the same reason we have them in the first place: by establishing long-distance connections and high-connectivity nodes, we help ideas spread much faster through our social networks. It's the same for a virus.
More math: Locally, early in an outbreak, the expected impact of a social event scales as the number of people times the number each interacts with. Roughly the attendance squared.
Therefore cancelling a 50 person event is over a 1000 times as important as cancelling a 1-1.
Therefore cancelling a 50 person event is over a 1000 times as important as cancelling a 1-1.
Of course this eventually becomes linear, at a 10,000 person event you can't possible interact with everyone. But the point remains that from an outbreak-spreading perspective large events are disproportionately more important than small ones.
Further, it does make a difference whether it's people who wouldn't usually interact or a bunch of people who'd all see each other anyway. But the most basic models (which work shockingly well, and remain a great approximation) assume no such structure exists.
