It's good for seeing all past notifications, but it could be extended in the future to include "notable topics" maybe stickies or "promoted topics", things like that. I'm not sure how many the drop down shows at any one time, so if you're mr popular, you might miss a few because it only shows the last 10 for example.
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I know these has been discussed before. Just wanted to have it in a thread as these are a bit important these days due to spam attack.