I don't about the aviation world in this respect but, for my day job of accountancy, this question is sometimes asked as it gives an indication of that person's experience. A chartered accountant could earn anything from £25k per annum to £250k and, as they don't have a log-book, proving their experience and knowledge can be tricky. Therefore, if you know what their salary expectation is and/or current salary it gives an indication of where their skills and abilities lie.
However, this fails when moving to new areas of the country or even different countries since standards of living can vary so much. For example, there can easily be a £10k per annum differential between some areas in the UK for the same job.
Hope this helps.
Cheers
Whirlygig