From Gavin Wignall
This post is part of a string of posts found here: Learn Blend in a Month.
When styling up a control, Expression Blend allows you to apply that style to other controls. This firstly allows file sizes to be kept down as the style is only written in the XAML once. Secondly this allows you to retain consistency across multiple controls set to the same style, when making changes to one, all controls set to that style will update automatically.
more here