2. An analogy
At this meeting, people are discussing a certain issue -- for example, getting new sidewalks installed downtown or getting new schoolbooks for the elementary school, or what to do about the new product your company is planning on introducing.
In the midst of the discussions on the new sidewalks or textbooks or product, someone walks into the room, interrupts everyone, then reads an advertisement for a local restaurant. He or she then leaves without waiting for comment.
Now imagine if this happened over and over again each time your group tried to hold a meeting. Every time someone tried to make a point, in walks some other stranger who reads an ad for some business that has nothing to do with the subject of the meeting. It would soon become rather difficult to hold effective meetings, wouldn't it?