CD4021 is a cascadable 8bit parallel to serial shift register (basically the inverse of the 74hc595).
Sometimes you'll end up needing more digital input than the pins on your microcontroller board can readily handle. Using a parallel to serial shift register allows you collect information from 8 or more switches while only using 3 of the pins on your microcontroller.
By the way, this is the IC you need for Arduino ShiftIn function.
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