Any time you need to make the same edits in multiple places. I use it to mass edit array items for example by using the highlight / add cursor to identical text feature in VS Code (Ctrl+D). I even use it to copy data out of the browser and mass edit it into data structures-- often there will be some pattern to the pasted data even if it's garbage, so being able to quickly set up multicursors around the patterns in the text makes this sort of task much easier, if you are luckily on how the data pastes out.
Multicursors are the number one editor innovation of the last ten years that developers should get comfortable with-- once you start to use them you won't want to use an editor without them.