It must fill some purpose though. I doubt it's entirely just marketing in most legitimate companies.
Effectively a/b testing job adds?
Or trying to get a range of candidates so they can find a good fit?
Let's say you have, like, 10 jobs to do but you're only going to hire two people (either loading them up with more work, or internally reshuffling responsibilities, probably a bit of both).
So you advertise for every role in your ideal team, then get the two candidates who plug the most holes, or look like the best fit.
I feel dirty suggesting it, but it probably happens.
I heard an argument that the fake jobs are actually to appease internal employees. "We know you are over-worked, but we are trying to get you some help. Look at these postings -too bad everyone who has applied thus far is a complete dud."
I've heard in the sales world, the way to hire two sales people is to hire three, then fire the bottom performer after a month or a quarter.
As a wild guess this may be part of the story a company tells itself. Every individual and company needs to tell themselves a nice "story" to feel good about themselves. In case of a company "damn, look how many jobs we're posting, we're growing and doing great" is a nice story to tell. Yeah the owners/manager know it's fake, the people writing the post know it's fake, people receiving applications also know it's fake, yet it still works. On paper officially they can tell each other how great they are doing. This is more likely how a large company would operate.
Another, more positive perspective from a small company I worked for is "ABH" (Always be hiring). That means always post jobs, and continue interviewing, because you might find an exceptional engineer for whom you'd make an exception and hire them. But at least in our case it was always an honest effort every time to sit down and evaluate the candidates, pay them to visit and interview face to face and such. It wasn't a game it indeed took quite a bit of effort on our side.