What is the "top-fill" function used for?

A humidifier is considered "top-fill" when its user manual indicates that you can fill it from the top. This way, you don't have to remove and carry the tank to fill it: a bottle of water is enough and this greatly simplifies the daily use of the humidifier. In the case of a "top-fill" humidifier, the tank is shaped like a bucket with a handle.