Many reasons. Kitkat could put information about their product, like nutritional content, ingredients, etc. They could also provide information about the histor

Many reasons. Kitkat could put information about their product, like nutritional content, ingredients, etc. They could also provide information about the history of the brand, the different varieties, specialty varieties, limited time offers from the past and present. Perhaps they could also provided things, like videos and images of past advertisements, especially if they were particularly funny or interesting. Additionally it's a great way for customer outreach. They could inform them about some new limited time variant offering, organize contests or challenges, and so on. For example a lot of these products also offer a kind of rewards system. After you eat your candy you can note down the little code on the side, and redeem it for free products, or to enter a contest. What better place to provide this than their own website? This would included stunts like what they have pulled on their website today. And of course you have the whole trademark protection and brand identity things too.