Maybe this is what you're saying, I'm not sure - my understanding was that the salt prevents reused passwords from resulting in the same hash. So, if I use 'password' and you use 'password' the salt+hash will be different. That way attackers can't just hash all the common passwords once and immediately associate them with different accounts.

You can also have a system salt(s) that are not stored with the database, so that if someone accesses the database they have to guess password and two salts, one of which they hopefully do not have via the same penetration.