The point is that computers are really fast now. There's no need to precompute anything, just crack hashes on the fly. The extra mangling slows things down.
If an attacker gets access to a database of user names/hashes, a salt prevents him from simply checking the hashes against a precomputed list because each hash is totally unique.
The attacker would have to recompute all hashes for each user using their individual salt.
At least that's what I remember from Computer Security ha
