By using a multiline tag, you can effectively override your templates base styles in an email template that you do not have access to modify the code to.
You have an email template that you have created but your client/end user wants to make a few changes on the fly like update all of the fonts and font sizes and colors. But does not have the budget to pay for those changes each and every time. So what do you do? Well if you have a well crafted CSS based email template, you can override styles by using a multiline tag pair to give limited access to the template’s styling.
I am sure there are cavets such as improper CSS styles, unclosed selectors, anything that has a declaration of !important in the styles located in the head tag, etc.