If you will use TextBlock with multiple Run tag inside, you will face a problem with space in between two Run tag.
To demonstrate it
<TextBlock Margin="250,0,0,0" FontSize="35">
<Run>Hello </Run>
<Run>World</Run>
</TextBlock>Use above code and it will give you output as "HelloWorld", while we want it as "Hello World".
So we need to use xml:space="preserve" to keep spaces, as written in below code
<TextBlock Margin="250,0,0,0" FontSize="35">
<Run xml:space="preserve">Hello </Run>
<Run>World</Run>
</TextBlock>