What does Control key do, other than its usage in terminal and right click?

Solution 1:

The Control key is used as a modifier for various purposes in different applications. In OS X, it is also used for Emacs bindings supported in many applications.

The Apple HIG (Human Interface Guidelines) that is recommended for application developers discourages the use of the Control key (the closest equivalent to the Control modifier key in Windows is the Command key on OS X):

As much as possible, avoid using the Control key. Because the Control key is already used by some of the universal access features—as well as in Cocoa text fields where Emacs-style key bindings are often used—it should be used as a modifier key only when necessary.

Here are some examples of shortcuts from Apple's KB on OS X keyboard shortcuts:

Sleep and shutdown shortcuts with Control

Command-Control-power button    Force your Mac to restart
Control-Media Eject (⏏)    Show restart/sleep/shutdown dialog
Command-Control-Media Eject (⏏)    Quit all applications (after giving you a chance to save changes to open documents), then restart the computer
Command-Option-Control-Media Eject (⏏)    Quit all applications (after giving you a chance to save changes to open documents), then shut down the computer
Shift-Control-Media Eject (⏏)    Put all displays to sleep

Finder shortcuts with Control

Command-Control-I      Get Summary Info
Command-Control-Up Arrow      Open the folder that contains the current folder in a new window

Application and other OS X shortcuts with Control (may not work in some applications)

Control-A      Move to beginning of line/paragraph
Control-B      Move one character backward
Control-D     Delete the character in front of the cursor
Control-E     Move to end of line/paragraph
Control-F     Move one character forward
Control-H     Delete the character behind the cursor
Control-K      Delete from the character in front of the cursor to the end of the line/paragraph
Control-L      Center the cursor/selection in the visible area
Control-N      Move down one line
Control-O      Insert a new line after the cursor
Control-P      Move up one line
Control-T      Transpose the character behind the cursor and the character in front of the cursor
Control-V      Move down one page
Control-Tab      Move focus to the next grouping of controls in a dialog or the next table (when Tab moves to the next cell)
Shift-Control-Tab      Move focus to the previous grouping of controls
Control-F1      Toggle full keyboard access on or off
Control-F2      Move focus to the menu bar
Control-F3      Move focus to the Dock
Control-F4      Move focus to the active (or next) window
Shift-Control-F4      Move focus to the previously active window
Control-F5      Move focus to the toolbar.
Control-F6      Move focus to the first (or next) panel
Shift-Control-F6      Move focus to the previous panel
Control-F7      Temporarily override the current keyboard access mode in windows and dialogs
Control-F8      Move to the status menus in the menu bar
Command-Option-Control-Comma (,)      Decrease screen contrast
Command-Option-Control-Period (.)      Increase screen contrast
Command-Shift-Control-3      Capture the screen to the Clipboard
Command-Shift-Control-4      Capture a selection to the Clipboard
Command-Control-C     Copy the formatting settings of the selected item and store on the Clipboard
Command-Control-D     Display the definition of the selected word in the Dictionary application
Command-Control-V      Apply formatting settings to the selected object (Paste Ruler Command)
Control-Right Arrow      Move focus to another value or cell within a view, such as a table
Control-Left Arrow      Move focus to another value or cell within a view, such as a table
Control-Down Arrow      Move focus to another value or cell within a view, such as a table
Control-Up Arrow      Move focus to another value or cell within a view, such as a table
Control-Space bar      Toggle between the current and previous input sources
Option-Control-Space bar      Toggle through all enabled input sources

Universal Access shortcuts with Control

Option-Control-F8 or fn-Option-Control-F8      Open VoiceOver Utility (if VoiceOver is on)
Option-Control-F7 or fn-Option-Control-F7      Display VoiceOver menu (if VoiceOver is on)
Option-Control-Semicolon (;)      Enable/disable VoiceOver Control Option-lock
Command-Option-Control-8      Invert/revert the screen colors (OS X Lion or earlier)
Command-Option-Control-Comma (,)      Reduce contrast (OS X Lion or earlier)
Command-Option-Control-Period (.)      Increase contrast (OS X Lion or earlier)