It is also common practice for companies to test the market for staff availability just to see what kind of resume and what kind of qualifications are available.
The responses to the add (or the lack of) gives a good gage of the available talent pool. This is important data, because if the offer out there is weak say for senior engineers, the employer will quickly match (and even beat)any offer coming to his own senior guy, because he knows the available replacement talent out there is zero.
It seems that when one does it, everyone follows suit, especially like you say, when people start seeing the end of the tunnel.
There are no jobs out there yet, the adds are fake for the most part, even from the repurable companies.
Employees are too busy and too poor (or lack the vision) to gage the market the way companies do.