What's the envelope inside mac's box? When I opened the box of my new MacBook Pro (damn it smelled so uniquely!!), below the laptop there is a black case, where the charger and another long (looped) cable (I don't know what this is suppose to do..) lie.

There is also a black packet, with the sign:


  Designed by Apple in California


Inside that, 2 booklets lie with Instructions, etc. and a black soft envelope, which is inside transparent latex(?). 

What is that soft envelope for?
The black piece of cloth is not an envelope, but a nicely folded display polishing cloth.
Apple mentions it in the manual:

Cleaning the MacBook Pro screen
To clean the MacBook Pro screen, first shut down
your MacBook Pro and unplug the power adapter. Then dampen the included cleaning
cloth with just water and wipe the screen. Do not spray liquid directly on the screen.

You can confirm the looks on this review. And the long cable is just an extender which you can plug into your charger.