Remember the crankshaft carries more than one piston, therefore the firing order is so arranged to ensure that there is a piston on the power stroke every time the crankshaft rotates.

Therefore, the power from this will be used to drive the crankshaft, in the process, also moving the other pistons that are attached to the crankshaft. Thank you for reading and for asking this meaningful question. I made a little explanation about strokes in my post on Radial Engines, you may check it out to know more.