I agree that it's pointless, but liking a product on Facebook are signaling to their friends that they like KitKat. I think social proof plays a role here: "will I be less cool if I don't show people how I like this thing some of my friends like too?"
A brand website doesn't quite have the same appeal of social proof. It takes a greater interest in a brand to visit its website than it takes to open a logo in a friend's FB profile.
Smart marketing agencies can take a dead-end brand that's perceived as low price or low quality, then socially market it to young hipsters and get them (and their friends) to like/follow it as a way to signal to their friends that they don't care about marketing or signalling and that they "get it", and thus in the process drive a renaissance for aforementioned product due to its newly found trendy fanbase.
Okay, makes sense, then correct my statement to: "The same people who participate on the KitKat Facebook fanpage." Now that's really pointless, unless you want to complain ;)
Seriously, I think the websites of candy or Coca Cola and similar consumer products are probably visited by people who want to participate in some sort of lottery or want to get some coupons. Maybe these people just check their favorite brands' websites once in a while for new coupons/prizes.
A brand website doesn't quite have the same appeal of social proof. It takes a greater interest in a brand to visit its website than it takes to open a logo in a friend's FB profile.