Being able to dynamically supply theming is a big one. For example:

    const Header = styled('header')`
       font-family: ${({ theme }) => theme.fonts.header};
       color: ${({ theme }) => theme.colors.primary};
       backgroundColor: ${({ theme }) => theme.colors.primaryBackground};
    `

...where the theme object can be swapped out on the fly for different sections of a web app.