Leaving the charger constantly plugged into an outlet causes it to overheat, which can accelerate wear on its internal components, such as capacitors.
If there is a sudden change in voltage, the charger connected to the mains may overheat and begin to smoke, which in some cases leads to a fire.
It’s also important to keep your home safe, especially if you have small children or pets. A charger in an outlet with a cord connected to it may present a risk of electric shock.