It's night and it's an emergency! The Joker is loose! Gotham needs Batman. But there isn't a cloud in the sky. How does Commissioner Gordon summon him?
Assuming he doesn't have a red phone line directly to Batman like in the old TV show, obviously.
The batsignal has often been described as a way to instill fear in the criminals, not as the most effective way to summon Batman per se. Certainly Batman doesn't just watch fhe sky all night long to wait for the batsignal to come up, he has ways to get notified without seeing it.