Notifications provide a set of actions for users to interact with. A couple of them ("X" and "...") don't have labels.
Although the symbols used are common and generally understood during user research, some extra clarification in the form of tooltip can be useful.
Based on these comments, it seems that tooltips could help confirm the purpose of such actions.
- "X": Mark as read
- "...": More options
Note that this applies to regular notifications, bundles and compact notifications inside bundles (e.., cross-wiki notifications)