When your child is lost it provides you with another indication of the child's location. This is especially important indoors where GPS is ineffective.
When you want to check on your child's well-being and care you will be able to understand the general atmosphere; if your child is laughing, singing, crying, being yelled at, etc.
Since the device is typically inside a pocket it is not always easy to understand every word being said.
Indoors, voice quality is better when the child is not moving around much. Outdoors it is more difficult to hear.
If you hear background noise only, it normally means there is no direct conversation with your child.