Hover over the tag and read its description.
Discussion questions are the heart of your community. For example: Should a certain question be allowed on the site? For what reason did a moderator lock a certain question? Are said actions setting a precedent? Be sure to present your entire case and background information/data so the subject can be discussed fully.
Plus there are questions tagged with question and also with answer. Remember this is meta, we can post lists, resources or have general discussions about, for or against something for the main site etc. Every post here needs at least one tag, discussion can house nearly all of this as more than likely there will be a general giving of opinions on what was raised.
It is a bit unusual for Stack Exchange to enforce (force) a tag selection like this, but meta fulfills a special — and essential — purpose among the sites. We use meta as The canonical repository for all feature-request, support, and bug reports on the system.
To keep it a trusted system, you must assure that one of these these tags are applied to all the posts that need them.
So you can't really say "you must tag everything with one of these tags… or leave it blank." You essentially have to include a none-of-the-above tag discussion to make sure a selection is made in each case — assuring the collection is complete.
Please note that for some reason, the current system requires every question on Meta to be tagged with at least one of the following four mandatory tags: bug, feature-request, support, and discussion. I do not see the point of this requirement, but I am not sure if this is likely to change.
Because the discussion tag does not seem to mean much, I prefer to use any of the other three mandatory tags whenever appropriate and never use the discussion tag with any of the other mandatory tags. That is, I treat the discussion tag as “everything other than bug reports, feature requests, and support requests.” But that is just my personal preference.