Why is PropertyChanged not always null?

Question

WPF

Standard implementation of the PropertyChanged event and its call method:

public event PropertyChangedEventHandler PropertyChanged;

private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
    if (PropertyChanged != null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

Accordingly, the event must be raised when the NotifyPropertyChanged method is called from somewhere.

Why is PropertyChanged not always null in such a check ?
Where is the subscription to the event?
For example, like this:

PropertyChanged += {какой-то метод};

Answer 1

The answer is in the comments.