Oh, you all weren't paid for your overtime? Sorry, you don't work for Hilton. You work for Rent-a-Janitor. And they just filed for bankruptcy and closed shop. Hilton had previously dropped their contract with them due to their unscrupulous practices. We have now contracted with a totally different company, Rent-a-Janitor 2.
(This is a made up example. I have no idea if Hilton does anything like this, but most all huge companies do it.)
It doesn't, you're literally paying a middleman to take away the risk and hassle of hiring an employee. That's the added value here.
Otherwise, you could just hire the worker directly for what that middleman is paying them. Working as a contractor, you get a pretty good idea of what your "rights" are worth in pure dollar terms.
It's sometimes difficult to fire employees as a way of lowering wages. The people left behind who are expected to make up for that through raised workloads and losing the experienced for green new hires might complain.
But if the employee is via third party, there is no hiring or firing involved, limiting the liability of the first company.
In that regard the sales pitch is similar to how Microsoft can sell a company email and word processing so that the company doesn't have to figure out how to write business software.
* X is available more cheaply from a third party supplier (who presumably has achieved some kinds of economies of scale) than our fully-loaded cost to deliver the same
* X is a commodity product where we do not feel we can differentiate ourselves by doing it in-house
* X is ancillary, but important, to our core business and we lack the expertise to do it well, so we will buy in rather than build that expertise
Not many hotels are bothering to drill their own wells these days, bearing in mind that water is available from utility suppliers - why would room cleaning service be immune from the same forces?
* Purchasing X through a third party allows us to circumvent laws and regulations meant to protect workers and/or consumers
You can argue that such agencies are more likely to disregard laws and regulations, but how would you fix that? With laws and regulations?
I can think of a couple different legal metaphors for this:
1. Something similar to banking's Know-Your-Customer could work... maybe call it Know-Your-Suppliers.
2. People are criminally liable for possessing child pornography even if they had no participation in the direct crimes of its creation. In other words: in this case there is no immunity just because the crimes happened somewhere "upstream".
For instance, how likely are your Fly-by-night employee or sub-sub-contractor to rat out the Fly-by-night? They may get a beating, their checks withheld, the Fly-by-night go under.
If it still happens, Big Publicly Listed company pays for it to go away in a settlement.
If it goes to court, they Big Company can say "we work very diligently to avoid this situation in the future and what happened is in no way indicative of our values".
Etc.
Agency work is extremely common here, especially on the lower level of the job ladder. Gig economy and contractors are common even in higher settings, such as software development.