To pick an instance at random: Confirmation emails for purchases made, especially if they have tickets as an attachments. If this email looks broken, customer thinks it's phishing or something, and might end up trashing it, just because styling is not supported consistently.
And I take issue with your argument of "marketing emails are bad." You may not like them, and that's fine, but there is a large part of the internet ecosystem that do like marketing emails- such as my mother who likes learning about deals/sales/coupons at her favourite outlets.