Page "CTIA and GTIA" Paragraph 15

** In Atari 8-bit computers, three of the pins are used to read state of the console keys ( Start / Select / Option ).

The fourth pin controls the speaker built into the Atari 400 / 800, used to generate keyboard clicks.

On later models there is no internal speaker, but the keyclick is still generated by GTIA and mixed with the regular audio output.