A hacker goes into a database and downloads a bunch of hashes and salts of users with knowledge of the hashing method. Say this is one entry that got downloaded:
Without the salt, the hacker simply computes 100M common passwords and their hashes and find matches. With the salt, the hacker simply computes 100M common passwords + 21022011 and their hashes and find matches. So, what's the difference for one user?
Sure, to massively crack hundreds of user passwords, the attacker would need to compute the rainbow table for 100M common passwords for each user. But the attacker can also be targeting just one user - maybe he knows the username of the company CEO and now with the database leak, the CEO's hash and salt as well.