Do I really need an acetone jar? WHY?

YES!
And the reason:
Residual resin and contaminates in the injector may cause the repair to discolor or may not even cure!
TRI GLASS resin is formulated not to discolor and remain clear as long as the equipment is maintained properly.

After using your windshield repair kit, unscrew the injectors, remove the orings, and soak them in acetone or denatured alcohol overnight. The next day the injectors will be clean and ready for use with your TRI GLASS windshield repair kit.