Battery-powered lights, which are wired in parallel, can also create a simulated "chasing" effect by alternating the polarity for each LED attached to the string, and controlling the positive and negative parts of the cycle separately.
By eliminating the need for extra wires, this reduces costs for the manufacturer, and makes the cords less bulky and obvious for the consumer to string on decorative items.
On cheaper sets, this causes strobing and prevents any of the LEDs from getting to full brightness, since both polarities share the same wire pair and cannot be active at the same time, meaning each can only be on during its own half of the cycle.
Better lights can adjust the duty cycle so that any unused "off" time on one polarity can be used by the other, reducing the strobing effect and making it easier to create color blends (such as orange, amber, and yellow from a red/green LED).
Without the antiparallel diodes in place, the voltage induced by the ESD event may result in the current following an unknown path that often leads to damage of the device.