The main purpose of a salt is to defeat a rainbow table. I.e. using a precomputed table of all possibilities to lookup more than one password at once. If the sa

The main purpose of a salt is to defeat a rainbow table. I.e. using a precomputed table of all possibilities to lookup more than one password at once. If the salt is different for each password, that guarantees that the work used to solve one password cannot be used for the other. Cryptographic randomness is not necessary, just uniqueness.