When you put a range of $100-120k, the very high quality candidate who won't accept an offer less than $150k doesn't apply.
Or, put another way:
> It could simultaneously be true that including a good salary range increases the number of applicants who are good fits for the role, and also true that including a good salary range decreases the average quality of candidates.
Part of "good fit" could be "willingness to accept compensation in range". If the job range says $10/hr, the average quality of applicants will go down because the MIT Ph.D.'s won't apply-- but you weren't going to hire them at $10/hr anyways.