A PBP file is a "PlayStation Base Package." It is essentially a compressed version of a standard PS1 disc image (usually found as .BIN/.CUE files). The primary advantages of using PBPs include:

CDRomance is widely considered the best curated site for retro enthusiasts. They specialize in "Eboot" (PBP) versions of games.

One of the most advanced standalone PS1 emulators, DuckStation handles PBP files with ease, offering upscaling and widescreen hacks. Managing Your Own Archive: PSX2PSP

The Beetle PSX and PCSX ReARmed cores both support .PBP files natively.