What was the PS1's Parallel I/O port used for?

I could not find any product planned or yet alone sold by Sony which uses that port. But there were 3rd party products which used it, for example cheat cartridges like GameShark:

These cartridges allowed to manipulate the game memory at runtime and thus allowed various forms of cheating. So apparently this port allowed to directly read and write the memory of the console.

Unfortunately (for Sony), there were also some cartridges which allowed to circumvent region lock and other DRM features. Which was against Sony's business interests. Apparently they could not prevent that without also breaking the intended functionality of that port. And it didn't look like they even had any serious plans for a product which used the functionality as intended. So they removed that port from later editions of the console.