In ancient Greece, the Agora (Marketplace) was the place where people came to sit in the sun, sip beverages and chit-chat, gossip, or just discuss the latest outrage perpetrated by the government.
The purpose of the Agora was not what it appeared to be. Rather, it was the place where, without import or content, people could begin to develop personal standards of trust in one another (i.e., they could engage each other, uncontaminated by the importance of the content of what they were discussing). After a while, you could count on one person to always be quick with the clever idea, another to be a willing "fair witness" able to compare presentations and form an unbiased judgement, yet another adept at defusing a tense situation with just the right quip..
When people make posts in a public forum, like a discussion database (DDb), those are echoes of the ancient Agora in modern technology. It allows people who literally know each other only through the ASCII characters (or the artifice of "avatars" in some media) they present to know the personality behind the mask. By moving to the less-critical topics, we allow humans to expose their values and their souls, becoming for a moment just a bit more real, just a tad more authentic. Then, we can rely on that pattern of reality and authenticity in the serious business for which the community is in formation.
Can it get out of hand? Sure, if not properly managed, it clearly can and does.. And other participants in the dialog, or around its edges, may object to the apparently extraneous traffic; those are early warnings that some threshold may be near. It is that balance between those who change the rules, and those who insist the rules (even the unwritten ones) be observed that is the dynamic tension that gives a community its life and vitality. Without both forces at play, the anarchists will burn the community out, and the serious-minded will stifle the liveliness of a good DDb, a good community. Healthy community is emphatically not about everyone being in agreement; it is about have healthy and vital dialog about things over which they have serious disagreement while never losing sight of the humanity of the people with the most dramatically opposing viewpoints.