Maybe for the perfect job, some people aren't that bothered what the salary is and sometimes if you know the industry you know what the going rate will be for the kind of job, but on the whole job descriptions are fairly vague and the possible salary range is pretty wide. Unless they quote it how are people supposed to know if the job is attractive to them.
e.g "Office Manager" can IME pay anything from around £18k to c. £50K
It must make a difference to the number of applications you get and the suitability of the applicants if you don't quote a salary? So why do do many businesses do it? Also, doesn't it mean you get applications from people who wouldn't accept it if they knew what the salary was, if you;re offering the bottom end?
If I was desperate for work, or the job sounded completely perfect I might apply when the salary isn't clear, but on the whole I ignore adverts where they don't say what the pay is. I can't be the only one?