Here’s a sample of some specific uses I’ve had over the years:
- Most blog posts are written in Whisk (I dislike writing/previewing in wordpress)
- At Apple, I’d often use tables in my emails to help communicate important points or results. I could quickly write up some
<table>code and the copy from the web view into the email and retain all formatting. - Playing with SVGs to figure out some of the viewports settings, how to handle background images, and modify filter effects
- It is often a playground for testing out new/upcoming HTML/CSS features that I check out for later integration into Hype
- It is where I write our release notes, which are just HTML lists
- It is also where I write our rich-styled emails (like the Welcome to the Whisk Beta Program one!)
- Sometimes it is easier to edit HTML that will be a part of a Hype document in an editor like Whisk than the popover Inner HTML editor
- All the tumult.com webpages are built from a homegrown static site generator that takes PHP and converts to HTML. So Whisk is the primary editor since I can fully edit live without even needing to run a web server or build first to preview the PHP. So I do use it for full web page editing, too!
I assume many of those won’t apply in your case since they have a lot to do with Hype development, but I’ve found it to be a continually useful tool for me 
.