​What is the function of this small bulb on the outside of a fluorescent light fixture that I saw in Japan?

While in Japan recently, I saw this fluorescent light fixture with a small bulb on the outside. When the light is off, the small bulb is unlit (as in the first picture). When the string is pulled to turn on the light, first the filament of the small bulb glows steadily for about 1-2 seconds (as in the second picture), and then the fluorescent tube flickers on and the small bulb flickers off simultaneously. When the string is pulled again to turn off the light, the small bulb remains unlit.

I have never seen a fluorescent light fixture like this in the United States. What function is this small bulb performing? Why do most fluorescent lights in the United States not have one of these?

Thank you in advance!