I think that the PSP or CPP credentials are useful to obtain if you want to be considered a security generalist rather than just a security/surveillance system technician or engineer. Studying for these tests gives you a broad perspective of security measures rather than just security technology.
Most job descriptions for security consultants or in-house security managers usually include a requirement to have either a PSP or CPP. Some jobs at security manufacturers also require a PSP or CPP. I believe that Assa Abloy was one of the first companies to require that all of their hardware application specialists have a PSP.
I personally poo-pooed the CPP credential for many years, and said that when my clients started asking for a CPP, I would get one. Sure enough, about ten years ago, I started seeing more and more client RFPs that specifically asked for the CPP credential. I studied for and passed the test on my first try, but it was honestly a little more challenging than I expected. After going through the process, I had a little more respect for the credential.
To be sure, the technology sections of both the PSP and CPP tests are hopelessly outdated, and the credentials are far from perfect, but until something better comes along, they are the most widely accepted certifications in the security industry, at least here in the US.