Unexpected Uses of Control Key

1. Open a folder in a new window (or not)

If you are not setting your "Finder Preferences" to "Always Open Folders In A New Window", command key + double click can always reverse your default setting. You can also press "Command-Control-O" to open a new window with your current directory.

2. Open a new window for the enclosing folder

When you are looking at the contents of a folder in the Finder, pressing "Command-Up Arrow" opens its enclosing folder.

3. Get a summary of info with multiple files or folders

Selecting multiple files and then "right click " or "command-I", it shows you the information of the files  with several windows. But with "Control", it shows you the information with a single windows. 

-- "right click" + Control

or

Command-Control-I