This is because the semantics of the light bulb shape have always been understood, consciously or subconsciously. Achille Castiglioni used it for this light that is designed to celebrate light, in the form of the opening the Flos showroom in Turin.
But it is not just the light bulb shape!
The playful base echoes a spool of cine film or audio tape, another shape that is full of meaning (almost literally if one considers the film or tape it could contain). But it is also practical: the cable can be wound around it and the plug held by the groove -- this is a light designed to be carried around. For it is not big:
You can see clearly in the photo above the frosted area of the bulb: turn that towards you and there will be no glare.
No wonder it is still in production 40 years later! It currently comes in orange and black: