The people you see in a desert with umbrellas are not using it for the rain, but for shade, the rain is the least of their problems.

Even in a desert, people still use umbrellas for protection from the rain:

https://lasvegassun.com/news/2016/jan/19/fast-moving-storm-b...

And the rain still causes problems, even (or maybe especially in) a desert:

https://nypost.com/2022/07/29/las-vegas-braces-for-more-rain...