RPCS3 has support for an IPC (Inter-Process Communication) protocol called PINE. This protocol allows external software to communicate with RPCS3 through a standardized interface.
The PINE protocol allows things such as reading emulator state and reading information about the current game. But most importantly, it allows reading from and writing to game memory. Of course your OS of choice probably already allows you to read the memory of another process, but you'd have to know where in the RPCS3 process the game memory resides. By using IPC you don't have to care about that, it's supported by RPCS3 and won't break in case the developers decide to move around the game memory.
Another use case could be reading the current game playing, and based on that, an external program would play songs from a specific music list. Or it could control your LED lights to create the perfect gaming environment for that game.
There are also the future possibilities, which are endless. To read more about PINE, visit the repository!