As with many things, all the suggested reasons apply, in different proportions for different people. The caffeine may help with vasodilation-related symptoms. It also may help with any other caffeine-withdrawal symptoms, if present. And it may create a slight new dependence – as with the other painkiller ingredients, which can contribute to 'rebound headaches' with persistent use. Adding an addictive yet legal substance to your product is rarely bad for repeat business.