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![](https://fry.gs/pictrs/image/c6832070-8625-4688-b9e5-5d519541e092.png)
AFAIK, they are used as relays.
From https://en.wikipedia.org/wiki/1-bit_computing#1-bit
Computers and microcomputers may also be used, but they tend to overcomplicate the task and often require highly trained personnel to develop and maintain the system. A simpler device, designed to operate on inputs and outputs one-at-a-time and configured to resemble a relay system, was introduced. These devices became known to the controls industry as programmable logic controllers (PLC).
See also the playlist linked in the other comment with more explanations:
1-Bit Breadboard Computer - Usagi Electric (YouTube)
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