They provide space for online collaborative work, so think of them as the equivalent of working together on a poster, or working together to draft a document. They're one of the clearest examples within Moodle of social constructionist
pedagogy in action, since the learners will collaboratively construct a 'knowledge artefact'.
I've used them very effectively for research, vocabulary or revision type homework, so I'll set up an intial page with a list of words to define, or concepts to explain, with [ ] around each of them which Moodle understands as a reference to another page in the wiki.
Moodle then displays these with a ?
next to each, clicking on that opens the editor and they type away their explanation, or provide more information.
Later on, others come along and add to or improve on what's there already, as well as, hopefully, learning something from what their peers have contributed.
Also used them for assembling the content of the school magazine, and for making school policies available for colleagues to review.