I'm a member of the RPS (I first joined as a student around 1966, but let my membership lapse until recently), and never sought these qualifications, ARPS etc. However in 2023 I was awarded their Progress Medal and Hon FRPS for science-related work ... I should add this was a complete surprise, totally unsolicited, and given for work I did nearly 50 years ago which turned out afterwards to have some impact.
I can't really comment on why people apply for ARPS etc, but would like to point out that the standard of photography, in diverse areas, exhibited by the RPS is amazingly high. As an example, look at any edition of their bi-monthly Journal, which has a truly outstanding section of prints. They are not solely "old fashioned" on the one hand or "modern" on the other, just a collection of really really interesting photos of exceptional quality.
So perhaps having their programme of qualifications is all part of raising the standard of photography?