WhatsApp works extremely closely linked to the mobile phone contacts, so that to send a message to someone it is normal to first add them to the contacts. However, this is not strictly necessary because it is possible to send WhatsApp messages without having the number in the phonebook.
Doing this has two advantages: you save yourself from adding people to your contacts that you’re only going to talk to once and also these contacts will not have permission to see the elementos you have set as “My contacts” in WhatsApp’s privacy settings.
