Fred Astaire used to sing in the rain.
Until he caught a bad cold, which became pneumonia.
It took him weeks to recover from it.
Most people can shake a cold pretty quickly, but a singer has to be extra-careful with their voice.
Not to mention that he was also a dancer.
These days, they’d do the rain with advanced computer graphics.
But you could still tell that he was dry from his face and clothes.
Sure, they could simulate that too, but not as well as actually getting him wet.
Which would give him a cold and pneumonia again.