Why can I bookmark my own question? Don't I already get a notification when something happens on my question anyway (I don't mean the actions on answers)?
Maybe you've asked hundreds of questions and...
There are a few you keep coming back to because they have useful but complicated answers and a bookmark is faster than searching each time.
There's one you've been meaning to alter/correct but you just don't have time right now.
There's one where you're not sure which answer is best right now so you want to remind yourself to look at them with a view to accepting and/or voting on them at a later date.
Or one of any number of reasons where you might want a reminder to revisit one of your many questions later.