Imagine you've been away for a weekend or a week and haven't been on SL. Whenever you log in again, surely you have some IMs waiting, some Inventory offers, some Group Notices. No big problem, you read it, check it all out and do what you came inworld for. Now imagine you have lots of IMs and Inventory offers every single day!
A while ago I saw a site made by a Linden with a small storyline about you being the Linden that gets a nightmare about never-ending IMs coming in (I couldn't find it anymore though, if anybody knows, post a comment). That was scary already, but it still appeared to be fiction.
So now Torley put up a video showing it's really everyday reality: