Why do cats open their mouths?

Sometimes, yes, they pant. This can also happen when they're kind of freaking out too. When I was taking my cat home from a cat motel, in his cat cage, he was panting like a dog. It was a hot day, and he was a bit uncomfortable aswell. I'm not sure if it's bad if it happens, but if it does, try to cool your cat down and calm it.