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the fact in my opinion is most of these companies depends on internal announcement & recommendations by their staff for employing friends, relatives, .etc
I think both , meaning the two cases applied as it make announce for the Job and same time interanl ask if any know any can apply for the Job , because he / she well know will the company policy work flow and will never and every submit any one unless he / she know will fit that place.
Usually companies advertise in news papers or publish through the internet for an open vacancy but recently recruiting agencies appeared and it is sometimes better for some companies as they don't have time to keep receiving calls and arranging interviews so they depend on these agencies for giving them the CVs and they choose from it. Of course there will cost for such help and the good thing for the company is that even if they choosed a staff member then they can replace him within3 months if he appeared to be not suitable for the job without paying the agency any extra charges.
So it is cheaper for them and avoid headache. These agencies take commission from both the hiring company and the staff member so they try their best to choose carefully in order to keep their business.