Design-time vs. runtime

gistfile1.xml

<common:LayoutAwarePage
    x:Class="Kodkaffe.App.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:Kodkaffe.App"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:common="using:Kodkaffe.App.Common"
    mc:Ignorable="d"
    DataContext="{Binding Main, Source={StaticResource Locator}}">

    <Page.Resources>
        <!-- This works, but why do I need the d:Source attribute?? -->
        <CollectionViewSource
            x:Name="subjectsViewSource"
            Source="{Binding Subjects}"
            d:Source="{Binding Main.Subjects, Source={StaticResource Locator}}"
/>
    </Page.Resources>