In addition to Andriana's point, it's a way of assessing greater meaning behind a person's employment history.
See Also
What Are Potential Employers Allowed to Ask in a Background Check?8 Questions Employers Aren't Allowed to Ask YouShould You Ask About Salary in an Interview? Here's How!Can my boss tell others about my salary?"Project Officer" roles, by way of just one example, could mean a huge range of things and I know of Project Officers on £21k and Project Officers on £75k and their roles, skills and qualifications are, naturally, very different.
So seeing the number associated with the job gives the recruiter an idea of whether the applicant is a suitable match.
Is it accurate? No. But it is a potentially helpful additional data point.
