Way useful. You can, for example, select all instances of a variable name, then rename it selectively from the middle of the selection if you want.

Also good to note: Command-F find also has a "Find All" option which can be faster than pressing Command-D multiple times, and also allows more complex non-word and regexp searches. From there you can use multiple cursors or replace the whole selection just by typing.

Brilliant editor, this one.