I often find job postings from third-party recruiters (a contracted personnel company, not an in-house recruiter) that do not list what company the job is for.
Why do they do this?
This is often because they don't want candidates to have the opportunity to work around them — to decline and then make direct contact with the company, cutting them out of the transaction and costing them their commission.
Recruiting companies deal with a lot of money for each position (20 to 35 percent of the starting salary, says this article), and typically won't get any commission if the hire was not found by the recruiter. At the same time, the company can't be expected to give up good potential hires that contact them directly.
This is why initial job postings sent to potential candidates by recruiters often don't include the company name. It puts the recruiter at risk of losing that contact if the candidate declines but then (good intentions or bad) goes directly to the company.
As for advertised job postings (gathering instead of hunting), it's the same thinking: they want to make sure their advertising money brings them the commission.
I was told this by a recruiter when I asked who the company was for before I agreed to have them send my resume to the company. He said they normally won't tell candidates until the candidate agrees to work with them.
We use recruiters pretty often and normally request that they keep the company name out of the picture until later stages if its for more senior positions.
If you are a listed company, it's pretty much a given that the company name won't be mentioned because even small things can affect market perception and you don't really want to take even minimal risk.
Even if you aren't listed but are relatively well known, most people won't want the market to know that their R&D director is leaving soon, etc, etc.
Looks like there are a variety of reasons to do so.
NickC hit one of the main reasons, the company does not want its best candidates cutting them out of the deal before they can even talk with the candidate. Another big reason is there are competitors out there that also would like to fill that position. Announcing who it is that you are seeking to place is essentially telling their competitors here is a business open to using contracting/placement firms. With a small amount of due diligence the competitor can come in and undercut the first company.
This places stress on the relationship with both parties of the originating deal. Many times these competitors will come in around the actual manager making the decision, and have the managers boss direct the manager to use the new company. This still happens. But by using good discretion the Initial firm can hopefully fill the position before any competitors can intervene.
Good contracting and placement firms spend quite a bit of time and money cultivating relationships with prospective customers. They spend a similar amount of time building relationships with their prospective candidates. They do not want to do anything that compromises their relationship with either.