> Why does that matter? It's not like rain means you didn't have to go to the wash

The car gets dirty again when it rains and when it gets dry again. I guess dust, salt, pollution and more is what gets mixed in and put on the chassi as it rains, falls from roofs and splashes, but can't say I've investigated deeply enough. Not the end of the world, just annoying it keeps happening.

Many people avoid washing cars just before rain to avoid spots, etc. Phoenix as an extreme example rarely rains and leaves everything filthy afterwards.