Green card job postings do this because it is a requirement to advertise the opening. So you tailor the job description specifically for the person you already employ on a immigrant visa.
You're probably referring to job postings used to justify H-1B (and similar) visa applications. Such visas are only supposed to be approved if the employer shows that no US person can do the job.
There's not really any green card job postings. But getting an H-1B can be the first step toward obtaining a green card (permanent residency) for some immigrants.
Honestly if the job ad surfaces someone who can do the job and doesn't need all that immigration rigamarole I'll take them in a flash. I'll save a ton just in lawyers and time lost.
And isn't that how it's supposed to work: hire local in preference to bringing in someone from outside? I'm an immigrant myself and I still think that's a good idea.
We had this in Australia a few years back. TCS employed a whole call centre to advertise jobs and interview many people for them so they could bring in their own overseas contractors because they could not fill the positions with local developers as they lacked the skills required. All of them.
