It seems like this practice of always having job listings posted, even if you don't intend to fill them, effectively hides the internal status of a company from the public. This signal -- "the company has more or fewer job listings compared to the past or compared to other companies" -- could have a marked effect on current employees, investors, analysts, etc. But the company may want more control over how these groups perceive it, so it masks this internal state by adding noise to the signal.