Nearly every sleep or focus app includes a rain sound, and it's usually one of the most played. That popularity isn't random, it comes down to the structure of the sound itself.
Random, but predictable
Rain sounds random to the ear, but it's actually fairly consistent in statistical terms. The frequency and intensity of the drops fluctuate within a narrow range, without sharp, sudden spikes. Your brain can classify that kind of sound as "no threat" and push it into the background quickly.
A natural sound that feels familiar
Most of us experienced rain from somewhere safe, indoors, rather than as a threat. That association sticks, which is part of why the same sound feels far more pleasant while you're sitting by a window on a rainy day.
It doesn't land the same for everyone
If rain is mostly associated with a string of miserable weather days for you, it might not bring the same relief. Sound preference is tied to personal history, so a texture that works wonders for one person might do nothing for someone else.
Which kind of rain matters too
A thin, distant drizzle and a heavy thunderstorm create very different effects. A light, steady rain tends to work better for focus, while something deeper and fuller is often preferred for falling asleep.