Create a Consistent Look with the Lightning Design System in Lightning Out and Lightning Components for Visualforce
Lightning Design System references are added to any page that’s using Lightning Out when the Lightning dependency app extends from ltng:outApp. In this way, Lightning Design System styles and resources are available “as expected” in your Lightning components, even when they’re used outside of Lightning Experience and Salesforce1. That is, your components can depend on Lightning Design System styles being available wherever they’re deployed.
In previous releases, Lightning Design System wasn’t included, making it challenging to create components that looked and behaved the same when they were used in different contexts.
If you don’t want Lightning Design System resources included automatically when you use Lightning Out, extend your dependency app from ltng:outAppUnstyled instead.