It’s simply because some companies policy requires the previous candidate payslip to decide on the offer.

Plus, secondary reason is actually to roughly gauge a candidates job scope. I mean qualifications aside for experienced employees we mainly look at your work experience and job description to determine if you’re a suitable candidate.

For a lot of roles there is no technical or practical tests done during the interview stage to really gauge your competency. So the interviewer is dependant on your resume and interview questions to gauge if you’ve really done certain tasks/job which is required for the job role.

Your old company will not share/reveal your JD as part of your employment verification. At most we can check your job title only and ask about your performance but not in detail what you do. Different companies may have different task to be done by same roles for example some companies business analyst only does documentation work and not actual project management. The pay you receive will usually reflect this.

Of course hr knows some companies pays people low and give low job titles and expects them to do tasks way above their pay grade. In these scenarios hr will refer back to the hiring manager and ask for his final decision after raising doubts or concerns. End of the day, if you share your payslip it is still up to hiring manager what he wants to give it’s his decision ultimately. Hr only makes recommendations. You can get more than 30% if the hiring manager insists. Although hr recommendation will usually be around 20-30% depending on the HR policy.