can you give me use cases of using tags? I have been using categories to organize my questions pool and I cannot understand how/whether adding tags will help me work more efficiently.
A question can have multiple tags but can only be in one category, is a reason to use tags. The attachment is from some of the questions that ships with my gapfill question type that illustrates the way Moodle 3.8 displays tags (it shows the first few tags which is very useful, especially when checking for questions that don't have tags)
Using tags as well as categories will help you create really detailed subsets of question items within your question bank, so the organisation of question items as a whole benefits. It is, however, quite an investment in time to add the metadata tags if you have a lot of questions.
This is another one of those situations where a "good and reliable" student comes in handy. Have them do the tags for you. Doesn't hurt to also remind them, Moodle logs just about everything, so you will know who to blame for inappropriate tags.