Handlebar weights obviously come in pairs. They owe their existence to the inevitable vibrations of your motorcycle engine.
They were, and still are, designed and fitted to reduce or even nullify these vibrations. However, the basic principle of 'what weighs more, vibrates less' does not stand in the way of aesthetics. Hence the striking eye-catching vibe some handlebar weights achieve give a motorcycle or scooter a subtle personal touch.
It makes sense that they come in an array of different colours, sizes and weights.