For example, if CheapoCorp is looking to replace reasonably well paid US workers with H1-B workers, they won't bid much for the visas. CheapoCorp isn't trying to get good talent from overseas. They're trying to save money, push down wages, have a workforce that they can mistreat (since they can't easily leave the company given their immigration status requires employment), etc.
By contrast, if a company is looking to hire great engineers or scientists from overseas because they're in a growing industry with a shortage of workers, they would be willing to pay a lot more to get the H1-B visas. They're not looking to save $20,000/year on someone's salary. They want top talent.
When companies are trying to replace their workforce with lower-paid foreign workers who can't complain (lest they lose their job and with it their immigration status), that's not what the H1-B system was designed for. It certainly is how some companies are using it. If you're on an H1-B and lose your job, you have 60 days to find a new job or you're gone. That's going to make you a much more compliant employee. You have little leverage to negotiate raises, you aren't likely to quit even if they're overworking you, they can pass you over for promotions and you'll quietly accept it.
But if the employer is competing in an auction for H1-B visas, they're more likely to be companies that are seeking out top talent rather than seeking out workers they can underpay and mistreat.