Before I begin with what this is actually about, some words for those who aren't familiar with AniDB, MyAnimelist, XDCC-Bots or at least one of those:
AniDB and MyAnimelist are both websites that let you create a list of anime titles. In the case of AniDB the focus lies on the anime episodes you have stored somewhere, MyAnimelist cares more about what episodes you saw.
XDCC bots are IRC bots that send you files on demand. They are a common distribution method for fansubbed anime and offer so called packlists, where the list all the files they offer.
Now to the real story: I'm a user of both AniDB and MyAnimelist, but I only maintain a list on the latter. I use AniDB of keeping up to date with newly released episodes of airing series. The problem is, that if you only want to see notifies for new releases of certain subgroups, you have to add episodes of the respective anime from that subgroup to your list. I, however, don't maintain a list on that site, so I'd get a ton of unnecessary notifies.
Now, since I get all my new episodes from XDCC bots I though I could make use of simply parsing their packlists, which was the first thing I did: write a JSON file with all neccessary info, a small PHP script to parse the packlist and a minimalistic site to display the output. With that I always had an up to date list of all released episodes of the series I watch and only from the subgroup I want them from.
The problem with that approach was, that I had to memorize the number of the last episode I saw for every of those anime. So I had to come up with something different. Since MyAnimelist always has the information on which episode of which anime I watched last it seemed natural to use that. Also, since I open the page with my list on it several times a day, it seemed to be a good idea to simply include the information about new episodes there.
Oh and by the way: since the actual request to the site you do not control (in my case the packlists) is not performed by the browser the JS is running on but by the PHP script on your server this method doesn't even violate the same origin policy more than JSONP itself — it's just a handy way to access data from other servers when you write userscripts. :)