When Sony brought PS1 classics to the PSP via the PlayStation Network, they developed a highly optimized internal emulator (POPS). To make it work, they included a streamlined version of the original PS1 BIOS. Enthusiasts eventually figured out how to extract this file for use in other emulators, such as RetroArch (specifically the DuckStation or SwanStation cores) and standalone mobile emulators like DuckStation or EPSXE. Why use the PSP 6.60 BIOS instead of a standard PS1 BIOS?
Many users on platforms like Reddit's R36S community consider it a "game changer" for performance, especially on low-powered handhelds.