I think alot of this is to get you and your particulars on their database. Then, if something does come up they have available personell.
There is one agency in asia that advertise for "immediate" vacancies and then after applying put you through the mill and nothing eventuates. When a vacancy does come up you are reminded that they have you on their books and some exclusivity on you. All BS of course. deters most from going to another agency though.