It is used as described here to mark a post as belonging to a given community. Communities aren't out yet, but they are not going to be a part of the blockchain consensus, thus anyone can "simulate" posting in a community right now, for example using Steeditor.

However, this field should be optional. I created an issue for that, so if anyone wants to contribute, this is a good first issue: https://github.com/jakipatryk/steeditor/issues/37