Those tables are used to store activity on questions for user favorites. So if you favorite a question, each answer/comment on it will get put in the qa_userevents table so it can be shown on your "My Updates" page.
The qa_sharedevents table is used when many users have favorited the same question. For example if 20 people favorited the same question, a new answer could add 20 rows to userevents, so instead it adds 1 to sharedevents.
See the big comment in qa-include/db/favorites.php for a full technical explanation. And yes they do get used - my site has 50k+ rows in each of the tables.