Question tags are metadata and can be used to filter questions within the question bank. For example you might assign a difficulty level, or a subject area. A question can only be in a single category but can have an arbitrary number of tags.

Updating the tags on a single question with 4 tags can be time consuming. It typically takes 9 clicks to update a single question. If you have a a category with 20 questions that is 180 clicks. And of course there is a good chance of making a mistake. With this plugin the whole task will typically take 10 clicks.