Well, The garaje editor 1.3.1 is a powerful tool to do things that you cant do in the game, here's a little list of things that you can do with the editor. (in 2.12 udpate):
-Change paint (from the paint menu or typing the hex code, it allows paint unpaintable cars in the game or use rare dlc chips)
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-Remove wings or bumpers via hex codes (im not sure if this works on all cars)
-Install performance parts (including hidden parts) via hex codes
-Create DLC cars (stealth, chrome lines, TC cars....) via hex codes
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-Add cars tickets and "hidden car" tickets (red bull x2010 JP flag doesnt work on 2.12) and DLC cars not included.
-Add parts tickets and "hidden parts" tickets
-Add paint chips (DLC paints not included)
-Add credits
-Modify A or B spec level
-Change number of days in the game
-Activate "secret menu"
-Activate text debug (i dont know what is it)
Things can you can do in 2.11 only:
-Swap chassis, engines and suspensions.
-Change HP and grip multiplier.
I guess thats is the most important things you need to know. (sorry my bad english)