Showing sallaries saves everyone's time and money, some people want higher wages, others are willing to go for the same, just want to change.. Really depends.
Showing salaries would seem to be a way of offering transparency. One worry might be people going for a higher paying role with just that intent. Why do you think companies might not show salaries?
Sorry about the relatively-crude language, but I don't know of a better way to put it that's still succinct.