Video calling will be rolled out to Android, iOS and Windows devices over the next few days.
WhatsApp has over one billion regular users worldwide and has strong roots in emerging markets (it has 160 million users in India for example). WhatsApp works well on slow networks and was built with mobile in mind. With this latest rollout, WhatsApp will make video calling available to a large group of people who don't have email addresses.
Sources and further reading: WhatsApp blog; Endgadget.