Mike Caulfield explains:
This new breed of Q&A sites is replacing more traditional wiki as a source of information, through a modality we’ll call “choral explanations” (a term based on Ward Cunningham’s claim that federated wiki is a “chorus of voices”)
These newer sites do not work like older Q&A sites. Older sites (e.g. Yahoo answers) are essentially transactional. A person with a question poses a question, and the answers below the question respond to it. When the original poster of the question selects an answer as sufficient or best, the question is closed, and people move on.
These older Q&A sites are simple variants of general forum architecture. And they get good results occasionally, but it also have the sort of problems a forum runs into — they tend to produce answers that look more like replies than generalized explanations. ...
Quora and Stack Exchange turned this process on its head. Instead of envisioning the Q&A site as a single-purpose forum, the new breed of Q&A site sees the model as half-wiki/half-forum. The question asked is like the title of a wiki page; it’s not transactional but communal. A question like “How can a person increase their chances in the lottery?” is the place where the community will store their collective knowledge one that point, and it is not owned by the person who asked the question.
See also "Chorus of Voices" by Kate Bowles:
What happens next in the participatory web is that our solitary and wandering search trails can become visible, shareable and open. Of course, they also get fed into the algorithmic mincer in the hope that a drop of profit can be squeezed out of enthusiasms we might be part of. And of course, open is also always open to abuse. But whatever predatory or corporate interests have an eye on our pathways, the fact is that we make them first by ourselves, and then we make them socially. We answer one another’s questions, generating spin-off curiosities of our own. We follow another person’s line of thinking. We’ve always done this in conversation, in a way that leans on presence and familiarity, and we’ve always done it as scholars (at least, until we took a wrong, wrong turn into the citation farm). Now on the open web we do it asynchronously with strangers: leaving a book on a bench, lemons on a fruit stand, a message under a bridge, a comment on a blog post, all for someone else to pick up.