Because fireplaces needs people to interact with fire, it is dangerous. Children and pets want to go to where it’s warm, but they don’t know the exact limits of where to reach. An adult may be burned if he or she is not careful. That’s why, these exists as protection.
Screens are there to protect against embers. Sometimes, the embers may pop out of the wood and out of the fireplace, insert, or stove. Screens prevent this from happening. They come in a standard size so it is easy to choose, but there are some which require custom fittings.