Mike Zamansky has posted number 28 in his Using Emacs video series. This one is about rectangle editing. As Zamansky says, you don’t often need this but when you do it’s really handy.
My problem with it is that because I use it so infrequently, I can never remember the key strokes. There are three solutions to that, one of which the video demonstrates: simply call the command by it’s name. If you’re using something like counsel, ido, or helm, that’s pretty easy because they will help you fill in the entire name correctly even if you don’t remember the exact name. Another possibility is to make a hydra for the rectangle commands. I’m always one day away from doing that. Finally, there’s which-key. Once you have that enabled, you can’t really use the “I can’t remember the proper keys” excuse anymore. If you can remember that Ctrl+x r is the prefix for dealing with rectangles, which-key will tell you what to do next.
The video is only 5 and a half minutes so it’s easy to find time to watch it. If you’re not familiar with editing rectangles, you should definitely spend the time so that when you need to use it, you’ll have an idea of what’s available.