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

I remember that the first set of PS1 models had a Parallel I/O port that was removed in later models.

For what is the port used for? I'm talking about the one on the left when viewed from the back, not the smaller one used for multiplayer games:

ps1

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What was the PS1's Parallel I/O port used for? - Womans Bridge in Buenos Aires, Argentine

What was the PS1's Parallel I/O port used for? - Aerial Photography of Container Van Lot

What was the PS1's Parallel I/O port used for? - Aerial View Photography of Container Van Lot

What is parallel IO PS1?

PSIO is a Flash Cartridge that plugs into your PlayStation's Parallel I/O Port. It allows you to instantly see your games running from an SD Card on real hardware making it the only way to genuinely play backups on your console. More Info: https://ps-io.com.

What is the difference between PSX and PS1?

The PlayStation (abbreviated as PS, commonly known as the PS1 or its codename PSX) is a home video game console developed and marketed by Sony Computer Entertainment.

What colors did the PS1 come in?

Original Model \u2013 While black consoles were the mainstay of SEGA and lighter colors for Nintendo, SONY went with a grey color for their first system.

What was the very first PlayStation?

PlayStation, video game console released in 1994 by Sony Computer Entertainment. The PlayStation, one of a new generation of 32-bit consoles, signaled Sony's rise to power in the video game world.

Why Was The PS1 So Successful?

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Images: Tom Fisk, JOSE LUIS VAZQUEZ, Tom Fisk, Tom Fisk