Send Whatsapp messages to anyone. Even if they are not in your contact list! You can try with your phone neighbour :)
Now with support for whatsapp for business

You just need to know the phone number, hit chat and your are into the conversation. Useful when you need to contact a buyer or a supplier.

Also, it's useful to hide your photo (and other privacy data) to the person you are chating with., just make sure to tune the Whatsapp settings > Account > Privacy to My contacts or Nobody