Por exemplo nessa questão o usuário marcou a resposta como community wiki. A dúvida é a seguinte, quando e porque devemos marcar uma resposta ou pergunta como wiki?
A good explanation can be found at The Future of Community Wiki (that was written in 2011, so now is the future for that time).
Accordingly to the link above, Community wiki functionality comes from the very beginning of StackExchange, when the editing system was not so improved as it is now. So it made more sense in the past but it still can be be applied in nowadays under certain circumstances.
In a nutshell, community wiki answers should be make when the answer is a result of a collaborative work among the network users, in oppose of an individual answer. It's not really common to happen, so in most cases you don't need mark an answer as a CW.
A newer post of CW can be found here: Putting the Community back in Wiki (this is from 2014 :) ), that says:
For the most part, we’ve turned it back into something that you can choose to use in cases where it lets you work together to create something wonderful:
Compiling a canonical reference
Consolidating the knowledge of the community
Encouraging the ongoing, active maintenance of a changing answer
Sometimes these are single, collaborative answers, other times questions where all contributions must be made in the form of edits. In all cases, the results are clearly that of a sum greater than the whole of its parts, a true community project.
Collaboration isn’t a rare thing on our network - the whole system, from posting and editing to voting to moderation, is based on the interaction of multiple users to produce a final product. Community wiki is for a special scenario, something built not by the expertise of one individual, then improved or iterated on by a few others, but rather something created by the concerted efforts of the community as a whole.
De início, quem coloca uma questão não tem a possibilidade de marcá-la como sendo CW (Community Wiki). Contudo, se essa pessoa desejar que a sua pergunta venha a ser transformada em CW, poderá pedir aos moderadores para o fazerem. Já quanto às respostas, há toda a liberdade de escolha do modo CW.
Do post The Future of Community Wiki, de agosto de 2011, no StackExchange Blog:
Community Wiki is primarily for Answers
If we haven't said this enough already, questions rarely, if ever, need community wiki. What about answers? We removed the ability for users to make a question community wiki, but left the ability for users to make an answer wiki.
The intent of community wiki in answers is to help share the burden of solving a question. An incomplete "seed" answer is a stepping stone to a complete solution with help from others; an incomplete question is a hindrance and an obstacle to getting a solution as no one understands the inquiry. It is in answers that the goal of community wiki, for the community, by the community, shows its truest colors.