Hi James,
The biggest benefit is that you can edit an HTML email's source if you have need to vs. the wysiwyg editor that you'll always have to edit via wysiwyg. As far as business use for this ability, it's really about how your emails render across different clients, it's damn near impossible to fix rendering issues universally in the wysiwyg but someone who is editing the HTML can make an email render properly across a variety of email clients and web browsers. Also, if you are trying to do any kind of responsive design work you'll be able to control that best via HTML vs. the wysiwyg.
Best,
Lauren