This is my first post here, so I’m feeling a little, ah, intimidated.
Some background: The last time I dabbled in this field was as a pimply-faced teenager. That was many years (OK, decades) ago - back then prices were in £sd.
Welcome to the wonderful world of electronics! 0.1uF capacitors are commonly used to decouple high frequency signals. They are often referred to as a “decoupling capacitor”.
Without them, voltage spikes could disrupt sensitive digital devices (leading to resets / unusual behavior).