They don't need to be harvested, but they will start new plants that are exactly like the mother plant. If you don't pick them off, at least some will start new plants when they drop off naturally.
There is no need to wait until they fall off to harvest, and they usually fall while the stem is still green. The longer you wait, the bigger the bulbils will be, but there will be a time when they drop off themselves. When they detach almost just by touching them, that would be the largest they will get.
