There are three primary reasons why you should never leave a charger in an outlet without your phone. I will explain why.

Regarding electrical safety, a plugged-in charger that has a cable that is hanging down might be a risk if you have small children or pets in your household. In the event that it is chewed on or tampered with, it poses the potential for electric shock.
Taking Preventative Actions

To limit the amount of power used and to prevent the danger of overheating, chargers should be unplugged when they are not being used.
Chargers should be inspected on a regular basis for any indications of wear or damage.

Children and animals should not be able to access chargers or the connections that connect them.

By being aware of these dangers and adopting some simple preventative actions, you can not only make your house a safer place to live but also prolong the amount of time that your chargers will continue to function.