The point of ValueConversionAttribute class?

You can potentially use the ValueConversionAttribute to determine what types are involved in the converters, and use that information usefully.Look at Piping Value Converters in WPF as an excellent example for the use of ValueConversionAttribute.

The example shows how multiple converter classes can be chained, and ValueConversion can be used to pass type info to next converter in line.

[ValueConversion( typeof( string ), typeof( ProcessingState ) )]public class IntegerStringToProcessingStateConverter : IValueConverter{ object IValueConverter.Convert(     object value, Type targetType, object parameter, CultureInfo culture ) {  int state;  bool numeric = Int32.TryParse( value as string, out state );  Debug.Assert( numeric, "value should be a String which contains a number" );  Debug.Assert( targetType.IsAssignableFrom( typeof( ProcessingState ) ),     "targetType should be ProcessingState" );   switch( state )  {   case -1:    return ProcessingState.Complete;    case 0:    return ProcessingState.Pending;    case +1:    return ProcessingState.Active;  }  return ProcessingState.Unknown; }  object IValueConverter.ConvertBack(     object value, Type targetType, object parameter, CultureInfo culture ) {  throw new NotSupportedException( "ConvertBack not supported." ); }}// *************************************************************[ValueConversion( typeof( ProcessingState ), typeof( Color ) )]public class ProcessingStateToColorConverter : IValueConverter{ object IValueConverter.Convert(     object value, Type targetType, object parameter, CultureInfo culture ) {  Debug.Assert(value is ProcessingState, "value should be a ProcessingState");  Debug.Assert( targetType == typeof( Color ), "targetType should be Color" );  switch( (ProcessingState)value )  {   case ProcessingState.Pending:    return Colors.Red;    case ProcessingState.Complete:    return Colors.Gold;    case ProcessingState.Active:    return Colors.Green;  }  return Colors.Transparent; }  object IValueConverter.ConvertBack(     object value, Type targetType, object parameter, CultureInfo culture ) {  throw new NotSupportedException( "ConvertBack not supported." ); }} object IValueConverter.Convert(     object value, Type targetType, object parameter, CultureInfo culture ){ object output = value;  for( int i = 0; i < this.Converters.Count; ++i ) {  IValueConverter converter = this.Converters[i];  Type currentTargetType = this.GetTargetType( i, targetType, true );  output = converter.Convert( output, currentTargetType, parameter, culture );  // If the converter returns 'DoNothing'   // then the binding operation should terminate.  if( output == Binding.DoNothing )   break; }  return output;}//***********Usage in XAML*************    <!-- Converts the Status attribute text to a Color -->    <local:ValueConverterGroup x:Key="statusForegroundGroup">          <local:IntegerStringToProcessingStateConverter  />          <local:ProcessingStateToColorConverter />    </local:ValueConverterGroup>

It is just an annotation.

MSDN:

When implementing the IValueConverter interface, it is a good practice to decorate the implementation with a ValueConversionAttribute attribute to indicate to development tools the data types involved in the conversion

I do not know what the "development tools" would do with that information...