I believe the way they work now is you complete the fire lanes to start one flashing. The next time you make a cannon shot, one of the other standup/drop targets (not the middle lightning bolt) is lit for a cannon jackpot. Collect cannon jackpot and insert is lit solid. It may not be what was originally planned, but the inserts are used.

The folks suggesting they do nothing aren't paying attention. Take the glass off and see for yourself.

The fact that the inserts sometimes flash and sometimes are lit solid should have given folks a clue that they aren't completely useless. The cannon jackpot is shown on the display just before the cannon rotates, along with any other jackpots available. You don't have to take the glass off. Just watch the inserts and the display.