Wpf UI update from other thread/task

Question

I make simple example window to demonstrate the problem. It is a complete example, you can copy and paste and try it by yourself. When I try to set Content property of Label control via Binding - UI updates nice in both cases (Task and Thread). But when I try to set Content property of Label directly to Label like _label.Content = "somestuff" - UI doesn't update. And Clear Button doesn't work anymore :(

XAML:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" 
        Height="350" 
        ResizeMode="NoResize"
        WindowStartupLocation="CenterScreen">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
        </Grid.RowDefinitions>
        <Label Grid.Row="0"
               Grid.ColumnSpan="3"
               Name="_label" 
               HorizontalAlignment="Center"
               VerticalAlignment="Center"
               FontSize="20"
               Content="{Binding LabelContent}"></Label>
        <Button Grid.Row="0"
                Grid.Column="3"
                Width="100"
                Height="100"
                Content="ClearLabel"
                Click="ClearClick">
        </Button>
        <Button Name="_button" 
                Height="50"
                Grid.Row="1"
                Margin="5"
                Content="ThreadWithBindings(works)"
                Click="ParallelThreadClickWithBindings">
        </Button>  
        <Button Name="_button1"
                Grid.Column="1"
                Grid.Row="1"
                Height="50"
                Margin="5"
                Content="ThreadWithoutBindings"
                Click="ParallelThreadClickWithoutBindings">
        </Button>
        <Button Name="_button2"
                Grid.Column="2"
                Grid.Row="1"
                Height="50"
                Margin="5"
                Content="TaskWithBindings(works)"
                Click="ParalleTaskClickWithBindings">
        </Button>
        <Button Name="_button3"
                Grid.Column="3"
                Grid.Row="1"
                Height="50"
                Margin="5"
                Content="TaskWithoutBindings"
                Click="ParallelTaskWithoutBindings">
        </Button>
    </Grid>
</Window>

CodeBehind:

public partial class MainWindow : INotifyPropertyChanged
{
    private String _labelContent;
    public String LabelContent
    {
        get { return _labelContent; }
        set
        {
            _labelContent = value;
            OnPropertyChanged("LabelContent");
        }
    }

    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
    }

    private void ParallelThreadClickWithBindings(Object sender, RoutedEventArgs e)  
    { 
        var thred = new Thread(t =>
        {
            LabelContent = "beforeSleep";
            Thread.Sleep(2000);
            LabelContent = "afterSleep";
        });
        thred.Start();
    }

    private void ParallelThreadClickWithoutBindings(Object sender, RoutedEventArgs e)
    {
        var thred = new Thread(t => Dispatcher.Invoke(new Action(() =>
        {
            _label.Content = "beforeSleep";
            Thread.Sleep(2000);
            _label.Content = "afterSleep";
        })));
        thred.Start();
    }

    private void ParalleTaskClickWithBindings(Object sender, RoutedEventArgs e)
    {
        Task.Factory.StartNew(() =>
        {
            LabelContent = "beforeSleep";
            Thread.Sleep(2000);
            LabelContent = "afterSleep";
        });
    }


    private void ParallelTaskWithoutBindings(Object sender, RoutedEventArgs e)
    {
        Task.Factory.StartNew(new Action(() => Dispatcher.Invoke(new Action(() =>
        {

            _label.Content = "beforeSleep";
            Thread.Sleep(2000);
            _label.Content = "afterSleep";
        }))));
    }

    private void ClearClick(Object sender, RoutedEventArgs e)
    {
        LabelContent = String.Empty;
    }


    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

I expected the same behavior from all of these methods. Two works fine, two - doesn't work. I try use both - Invoke and BeginInvoke methods - all the same. Can anybody explain me the reason?