i want to know when i should use 'group' (ctrl+G) and union from pathfinder. i think They are producing same thing .Is there any particular reason for creating two process/command which produce same result?
When you make a group you make a container which holds together the selected objects. The individual objects can still have separate appearance and they can be moved around. You can later choose to ungroup to revert to individual objects.
When you use unite from pathfinder you combine the selected objects to one vector object. The individual parts can't have their own appearance and they can't be moved separately anymore. This action can't be reverted later.
When to choose one or the other is hard to say. It depends on the task at hand. I would say that if you can avoid uniting it leaves your artwork more editable, so I in this case I would probably just choose to group.