I've known this for a long time, but never found a use for it.
In the example provided it would've been much easier and faster to write down the first line, duplicate it down using Shift + Alt + Arrow Down (iirc) and type the remaining symbol.

Multi-cursor is useful in other situations though. Like when you want to select and replace a few cases but not all. For example, change some new lines to spaces or vise versa.