What does Control key do, other than its usage in terminal and right click?
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&nbsp;&nbsp;&nbsp;&nbsp;Force your Mac to restart
  Control-Media Eject  (⏏)&nbsp;&nbsp;&nbsp;&nbsp;Show restart/sleep/shutdown dialog
  Command-Control-Media Eject  (⏏)&nbsp;&nbsp;&nbsp;&nbsp;Quit all applications (after giving you a chance to save changes to open documents), then restart the computer
  Command-Option-Control-Media Eject  (⏏)&nbsp;&nbsp;&nbsp;&nbsp;Quit all applications (after giving you a chance to save changes to open documents), then shut down the computer
  Shift-Control-Media Eject  (⏏)&nbsp;&nbsp;&nbsp;&nbsp;Put all displays to sleep
  
  Finder shortcuts with Control
  
  Command-Control-I &nbsp;&nbsp;&nbsp;&nbsp; Get Summary Info
  Command-Control-Up Arrow &nbsp;&nbsp;&nbsp;&nbsp; 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 &nbsp;&nbsp;&nbsp;&nbsp; Move to beginning of line/paragraph
  Control-B &nbsp;&nbsp;&nbsp;&nbsp; Move one character backward
  Control-D&nbsp;&nbsp;&nbsp;&nbsp; Delete the character in front of the cursor
  Control-E&nbsp;&nbsp;&nbsp;&nbsp; Move to end of line/paragraph
  Control-F&nbsp;&nbsp;&nbsp;&nbsp; Move one character forward
  Control-H&nbsp;&nbsp;&nbsp;&nbsp; Delete the character behind the cursor
  Control-K &nbsp;&nbsp;&nbsp;&nbsp; Delete from the character in front of the cursor to the end of the line/paragraph
  Control-L &nbsp;&nbsp;&nbsp;&nbsp; Center the cursor/selection in the visible area
  Control-N &nbsp;&nbsp;&nbsp;&nbsp; Move down one line
  Control-O &nbsp;&nbsp;&nbsp;&nbsp; Insert a new line after the cursor
  Control-P &nbsp;&nbsp;&nbsp;&nbsp; Move up one line
  Control-T &nbsp;&nbsp;&nbsp;&nbsp; Transpose the character behind the cursor and the character in front of the cursor
  Control-V &nbsp;&nbsp;&nbsp;&nbsp; Move down one page
  Control-Tab &nbsp;&nbsp;&nbsp;&nbsp; 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 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the previous grouping of controls
  Control-F1 &nbsp;&nbsp;&nbsp;&nbsp; Toggle full keyboard access on or off
  Control-F2 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the menu bar
  Control-F3 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the Dock
  Control-F4 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the active (or next) window
  Shift-Control-F4 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the previously active window
  Control-F5 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the toolbar.
  Control-F6 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the first (or next) panel
  Shift-Control-F6 &nbsp;&nbsp;&nbsp;&nbsp; Move focus to the previous panel
  Control-F7 &nbsp;&nbsp;&nbsp;&nbsp; Temporarily override the current keyboard access mode in windows and dialogs
  Control-F8 &nbsp;&nbsp;&nbsp;&nbsp; Move to the status menus in the menu bar
  Command-Option-Control-Comma (,) &nbsp;&nbsp;&nbsp;&nbsp; Decrease screen contrast
  Command-Option-Control-Period (.) &nbsp;&nbsp;&nbsp;&nbsp; Increase screen contrast
  Command-Shift-Control-3  &nbsp;&nbsp;&nbsp;&nbsp; Capture the screen to the Clipboard
  Command-Shift-Control-4 &nbsp;&nbsp;&nbsp;&nbsp; Capture a selection to the Clipboard
  Command-Control-C&nbsp;&nbsp;&nbsp;&nbsp; Copy the formatting settings of the selected item and store on the Clipboard
  Command-Control-D&nbsp;&nbsp;&nbsp;&nbsp; Display the definition of the selected word in the Dictionary application
  Command-Control-V &nbsp;&nbsp;&nbsp;&nbsp; Apply formatting settings to the selected object (Paste Ruler Command)
  Control-Right Arrow  &nbsp;&nbsp;&nbsp;&nbsp; Move focus to another value or cell within a view, such as a table
  Control-Left Arrow  &nbsp;&nbsp;&nbsp;&nbsp; Move focus to another value or cell within a view, such as a table
  Control-Down Arrow  &nbsp;&nbsp;&nbsp;&nbsp; Move focus to another value or cell within a view, such as a table
  Control-Up Arrow  &nbsp;&nbsp;&nbsp;&nbsp; Move focus to another value or cell within a view, such as a table
  Control-Space bar &nbsp;&nbsp;&nbsp;&nbsp; Toggle between the current and previous input sources
  Option-Control-Space bar &nbsp;&nbsp;&nbsp;&nbsp; Toggle through all enabled input sources  
  
  Universal Access shortcuts with Control
  
  Option-Control-F8 or fn-Option-Control-F8 &nbsp;&nbsp;&nbsp;&nbsp; Open VoiceOver Utility (if VoiceOver is on)
  Option-Control-F7 or fn-Option-Control-F7 &nbsp;&nbsp;&nbsp;&nbsp; Display VoiceOver menu (if VoiceOver is on)
  Option-Control-Semicolon (;) &nbsp;&nbsp;&nbsp;&nbsp; Enable/disable VoiceOver Control Option-lock
  Command-Option-Control-8 &nbsp;&nbsp;&nbsp;&nbsp; Invert/revert the screen colors (OS X Lion or earlier)
  Command-Option-Control-Comma (,) &nbsp;&nbsp;&nbsp;&nbsp; Reduce contrast (OS X Lion or earlier)
  Command-Option-Control-Period (.) &nbsp;&nbsp;&nbsp;&nbsp; Increase contrast (OS X Lion or earlier)