If you want to use fsck, then Disk Utility will do that just fine in a graphical manner. The problem is that Apple's fsck does not fix a bunch of problems that can occur with HFS+ (neither does Norton's Disk Doctor, for that matter).

Anyway, here's my story, that should explain why DiskWarrior is a Good Thing:

I was running qmail and leafnode on 10.2: between them, these two programs create and delete a huge number of tiny files. Pretty soon, I started to see lots of kernel panics: one or two a day - this actually looked a lot like bad RAM to me at first, but it eventually became clear that the problem only ever occured when Mail was looking for new mail. Running fsck as single user listed a bunch of problems that fsck said it had decided not to fix (too much effort or something, I guess).

At this point I bought Norton. Disk Doctor improved things: suddenly, the panics were only coming once a week! Woo, at least the people who were still using Windows 98 weren't laughing at me any more. Actually, half the panics now were caused by Norton itself, which insists on installing a couple of dodgy kernel extensions - however, the rest were still happening.

Norton was generally proving a waste of money. Speed Disk didn't work at all (too many directory entries - hooray!) and that undelete thing was basically proving a crash factory, and even after I had spent an hour removing its various tentacles, once a week, Mail was causing a crash. Hrmph.

At this point I ordered DiskWarrior: rebuilding the entire directory sounded a bit drastic and risky to me, but these kernel panics were starting to annoy me. I was sufficiently impatient that I didn't wait for the DiskWarrior CD to be shipped to me (which took 6 weeks to the UK), but built my own very slow boot CD, and (eventually) ran Disk Warrior from there. The directory fragmentation was something like 75% at this point.

So, reboot, half expecting the disk to be totally dead, aaand... Well, it wasn't totally dead. In fact, it felt a bit snappier than before. You might recall that I mentioned leafnode before. The last thing its fetchnews utility does before finishing is 'fork a child process to update XOVER information', something that before I applied DiskWarrior took around 3 minutes - afterwards, it was instantaneous (this always felt wrong, having run leafnode for years on Linux before).

DiskWarrior had recovered one file, too. Unsurprisingly, this was a half-written email that qmail had been processing when I had had a power failure. Evidently, it lurked somewhere in the file system, randomly surfacing to pull yet another hapless kernel into the murky depths of a panic.

Under 10.2, future power failures seemed to cause similar problems fairly regularly. Now I've got 10.3, they happen less frequently, but DiskWarrior still periodically finds things to fix.