The fit kit is used by snowboard shops as part of the boot fitting heat molding process. The foam toe caps ensure that you get the all important wriggle room.
Pop the correct size toe cap on your toes and then put on the included sock. Pop your feet into the heated boot liner, tighten the liner and the boot and stand in place for 15 minutes.
If you pop on YouTube you can see ways of heat molding your liners.