I bought a 160GB external some months back and have not regretted it, DO IT. I've seen some advertised for about 360 canadin or so that go up to 500GB or something, they amaze me very much. My bandwidth isn't really high enough to accumulate anything in that amount of time though. Even now my 160 is only half full. I fully expect to have a new computer by the time I'll actualy need the extra space.
Beyond the extra space, the advantage is that you can use it as a place to backup your computer data if you're reformatting. That, or ifyou need to leave the place in a hurry, like in a fire. It's also something you can take anyway (comp hard drives are too bulky) like if you had to go to a lot of business meetings or wanted to access or share stuff on a friend's computer.
What I think would be cool is if they designed an external in a fireproof box (I have this for important papers like birth certificate) so that it could be connected, but wth an easily contractible and sealable cable hole so you could quickly do it and leave quickly if there's a fire. Fires scare me.
I use external for PDF, image collections, videos (fansubs!), and also for game installation files. There are a lot of awesome free games out there. You can easily forget about them if you don't install them, but what if your computer isn't good enough? Download the installation files, store them on an external, and then when you get a better comp you won't need to wait to download them, just install and get updates and it's ready! That's what I've done, I think I have 50 game install files from those free MMORPGS ready to play. They get supported by people donating or by buying items with real cash no one else can get.