A proper KDF is more important if you are trying to break a specific user's password. It will simply take too long to try any reasonable number of attempts. However, in the event that you have a large database leak individual salts become just as important. When looking thousands of users, unsalted hashes allow attackers to start with the most common passwords used and compare them to the entire leaked database in one try.

Yes, it is!

Since DDG does it quick, I'll use MD5 as an example, but don't ever use MD5 as your hash!

An attacker already knows a user's plaintext password is "pass123" and no salting has occurred. He can search for "32250170a0dca92d53ec9624f336ca24" in the database and find 5 other users that have the same hash. He now knows that those 5 people have the same password.

Meanwhile:

    Johnpass123: e3445c82086cff25a79dcbbe59b569d1  
    Mattpass123: fd6d563970fd6ead6391a997a5e06d80

Moral: It's not about how hard it is to crack. It's about comparisons of hashed values without cracking necessary.