Is binding IsEnabled optional when binding Command?

I noticed that the following code:

<Button Content="_Timbres..." 
    Command="{Binding Path=ShowTimbresCommand}" 
    IsEnabled="{Binding Path=CanExecuteShowTimbresCommand}"/>

behaves equally as:

<Button Content="_Timbres..." 
    Command="{Binding Path=ShowTimbresCommand}">

Meaning that the CanExecuteShowTimbresCommand is automatically bound to the IsEnabled property. Is that true and why?