c# – 事件 – 处理程序与直接访问？为什么？

public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(String propertyName) 
{ 
    PropertyChangedEventHandler handler = PropertyChanged; 
    if (handler != null) 
    {
        handler(this,new PropertyChangedEventArgs(propertyName));
    }
}

VS：

public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(String propertyName) 
{ 
    if (PropertyChanged!= null) 
    {
        PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
    }
}

为什么我总是看到人们创建将PropertyChanged分配给“处理程序”而不是仅仅使用它？