The android app store uses google wallet for buying apps , which is why they probably use it in Whatsapp.
Whatsapp is free on the iPhone, so there is a trick where you can install WhatsApp on an iPhone using your details, then use the details on your android phone and it should stay free forever
So if you have a friend with an iPhone who doesn't use whatsapp, it's a way around having to register a Google wallet account.