1) Create a chat with yourself. Why?
* You can use the chat with yourself to save a grocery list, those memes or gifs to share at the right time. etc.
* If you use the desktop version of WhatsApp, a conversation with yourself can be an easy and convenient way to send documents, photos and videos from your computer to your smartphone.
* You can use this chat to test new WhatsApp features.
* You can use this chat to test an external share you want to make in WhatsApp before sending it to your friends or groups.
2) Create a conversation with a number that is not registered in your contacts. Why?
* Useful for ordering from multiple budgets for example. Then you enter only the number that interests you in your contacts.
* Another example would be in case you are in another city and want to order a pizza from WhatsApp. You probably will not need that number again, so there's no need to register it in your contacts.