What is the origin of the idiom "comfortable as an old shoe" and when to use it?

The origin of the phrase "comfortable as an old shoe" is not definitively known, but it dates back to at least the 18th century. The expression likely draws from the idea that a well-worn shoe molds to the shape of the foot over time, becoming more comfortable with each wear. This idiom can be used to depict a person's comfort in a familiar setting, such as being in their own home or engaging in a favorite activity.