The only people wondering why FM needs a hard drive are people who have never played the game. Unfortunately, since the game never really got anywhere outside of Europe until Sega started publishing it a year or two ago, that means there's really quite a lot of people who haven't played it. Fortunately, there's an easy way to remedy this. Go and download the Football Manager 2006 demo and install it on your PC. Then play with it a bit, and look at the size of your saved game folder. Also consider the fact that there are regular downloadable addons which update the player/stats database to reflect the current state of the sport in real life. After that, I don't think you'll have any confusion about why the hard drive is necessary.
To give you an idea of just how popular the Football Manager (originally Championship Manager) series is over here, here's a snippet from a recent Eurogamer article:
Eurogamer said:
Championship Manager 4, the game which introduced match highlights to the series. Fact fans might like to know that it's the UK's fastest selling game ever, with more than 100,000 copies shifted in the first weekend it was released. Fact two: the game has been cited in three divorce cases.