noise generators,
motion sensors (for example detect if door opens by contacts being separated or infrared beam being broken) ,
on/off/wakeup controllers (hold button x seconds to turn on or off main device or a relay),
switch between battery and dc input, enable charging of battery when dc input plugged in
if it has adc with internal reference, maybe battery monitor ex chip reads battery voltage and lights up 1 to 4 leds, or blinks color of a led (red. yellow, green) or beeps every n minutes if battery goes below some threshold.
replacement for old chips that are no longer made (like let's say 4 bit or 8 bit input to 7 segment hex digit),
conversion between interfaces for example convert some proprietary display stuff to 4bit/8 bit lcd display (2x16 / 4x20)