[stephen_g]

DACs and ADCs are frequently used for digital communication - for anything RF/wireless they are a key part! Some high-speed SerDes receiver designs are even digitising using fast ADCs and DSP techniques internally now to do things like equalisation in the digital domain too, which is interesting (e.g. [1], [2]). Previously, slower (like 25Gbps and less) SerDes have tended to be purely analogue circuits until near the very end.

1. https://www.youtube.com/watch?v=OY2Dn4EDPiA

2. https://ieeexplore.ieee.org/document/8778136

[titzer]

I think the GP meant that you wouldn't normally want to try to use an ADC for digital communication because the general-purpose ADCs in, e.g. microcontrollers are either overkill (10 bits just to detect a level!?) or too slow (max sampling rate) to do high-performance I/O. You'd be better off using an existing protocol with just two voltage levels and the GPIO pins. You can make an analog circuit to transform the two voltage levels you do have into the (typically 0-5V) levels detected by the GPIO on the microcontroller.