It's not required in most locations, but often times best practice.
When you don't include pay range, employees may not know if they can accept the job, and therefore be reluctant to apply.
Also, you don't want someone to get part way through your hiring process only to find out that the pay won't work for them.
If you are posting in a location that legally requires a salary be posted your job will not appear on any job boards until a salary is added. Where it is required today is listed under "Does a job post have to include pay?" on our free job posting sites page; that list is not complete, the rules change and go down to city level in places, so check your state's own guidance too.