Earlier quoted context omitted.
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 th…
Personally while there are some task that's easier with multiple cursors, I have personally yet to find any use case where regexp replace in selections doesn't work as well.
As for regexps, the syntax in Emacs is hell, but I do know Emcacs has very powerful edit/replace tools.