I think it's to the benefit of the vast majority of job seekers, reducing the economic power imbalance between employer and employee, and I'd personally consider that a worthwhile reason to pass a law in and of itself. In addition, I suspect it's also to the benefit of most employers, many of which are not actually large enough to squeeze any appreciable benefit out of salary secrecy but who would gain from having more information about labor prices.

To put it more simply: the socialist in me thinks it's good for workers -- the liberal in me thinks it's good for fairer competition in the labor market. Win-win.