As a real Debian unstable addict, for a long time, I wanted to have real time notifications when upstream repository updates. So I can immediately check what's new and, time permitted, do the upgrade right away.
Read more here
Let me also say that I know that there are GNOME applets that do the similar task, sitting in the tray and checking for updates. But for many reasons, I wanted my own solution. First of all, I don't like programs taking place in my GNOME panel (or system memory), when there's no pressing need for that. Second, I wanted much faster response. Finally, doing some perl scripting from time to time is good for one's karma.