What’s the overlap of “really great candidates” and “people who can’t afford $X application fee”? If the fee is a couple hundred, sure. But $20? What percent of bona fide job seekers who are good fits can’t afford $20?
Anyone that doesn't get hired at the first company they apply for? $20 isn't much if it's a single application, but I suspect most employees need many hundreds of applications to find a company that'll hire them. Even more so with fake jobs, and fake jobs incentivised to be such due to the idea mentioned in this post.
$20 may not be much, but $200 or $2000 might be, especially if the candidate was recently homeless, has been laid off for a while, is from a poorer background in general or has other things to pay for. That great candidate might be a great candidate for your job, but may also be one for many others, and see those as a better deal.
