It is used to ward off evil spirits.
These are some Feng Shui rules for housing, derived from the Bagua Mirror. Feng shui masters believe that the ability of mirrors to dispel evil spirits is first-class. Walking in the streets and alleys, it is easy to find Bagua mirrors hanging on the eaves and walls. When choosing a Bagua mirror, choose based on its purpose. A convex mirror can suppress evil spirits in the house, while a concave mirror can attract wealth and bring blessings. A Tai Chi Bagua mirror can turn things around and adjust feng shui. Since it is hung for a long time, choose pure copper material as much as possible. Pure copper itself can dissolve evil spirits. Convenient for permanent use. Bagua flat mirrors are mostly used to block undesirable outdoor architectural shapes, such as road clashes, angles, temples, Yin Qi (relics, cemeteries, hospitals and old houses in disrepair, etc.), and foot-cutting (the door of the house is close to the road or in the living room). The balcony is quite close to the elevated road, etc.) and the fire is evil (high-voltage lines, substations, chimneys and transmission towers are found near the house). The Bagua mirror flat mirror can be made of a variety of materials, among which the bronze Bagua mirror has the best Feng Shui effect.
Of course, these are superstitions passed down from ancient times. They have no scientific basis. It depends on the individual whether they believe it or not.