Lately I've seen some fields that look like:
What is this?
This help page describes some about the feature. It used to be much more prominently documented and encouraged, but in the last few years the entire network has kind of backed away from the feature. There are some cases where it is useful but most of the time it just gets in the way and it doesn't accomplish in practice what people hope it might.
On this site I think it should almost never be used. I do qualify that with "almost" but I can't think of a single instance where it has actually been a good thing. Most of the time it gets used when people are too lazy to put together a full answer and just throw something out there "for others to improve on". It is more often used as an excuse to distance themselves from poor posts than encourage collaboration on good ones.
Theoretically I could see a number of people collaborating on a post on or offline by prearranged agreement to work together, then posting the result and disowning the rep gain for whatever person actually posted it, but that isn't what most people do. Remember that all posts are editable by the community anyway, and that process has better accountability in the form of a review stage without using the CW feature.
If you see it in use and there isn't an obvious reason for it (the post isn't already a collaboration), feel free to flag those posts for a moderator to remove the status.