It's an easier way to keep excess water drained. It allows the substrate to get rinsed and drained, rather than having stagnant water, and bacteria and ammonia overload.
Some people have a hole drained in the glass bottom by a glass cutter to allow drainage. This cannot be done with tempered glass, but can with regular glass. Then you would have a collection device such as a bucket under the hole that you could empty.
I mist enough to keep the humidity at the proper settings for the particular frog living in the vivarium.