It can also help if you’re doing something like applying a certain stylesheet, color, or format to more than one selection. But like its better-known sibling Control-Z, sometimes it can fail.
For me, it was acting super weird. Sometimes it would only repeat part of what I had typed, so that “$24,783” would become “$24.” Other times it would totally fail. I noticed that if I hit Control-Y within a millisecond of performing the action I wanted it to repeat, it would work. But if I waited a few seconds, nothing.
It turns out that SnagIt, an app that’s otherwise awesome, had set itself to automatically turn on whenever I fired up Windows, and was interfering with Word’s operation. The only way I noticed that Snagit was operating was a teeny red oblong that appeared at the top of my screen. The fix was to fire up SnagIt and change the preferences so that it didn’t automatically start with Windows.