This is a random trick I discovered when I changed my WhatsApp number. As the heading says, you can talk to yourself, send some useful messages, notes, photos videos, etc. to yourself to keep them arranged in a regular way. It is same as emailing yourself. Normally, you can’t message yourself as even if you have your own number saved in contact, WhatsApp contacts doesn’t show your name in its list but if you have changed your number even once using the above trick, you can find your old number available in WhatsApp contacts list. Open it and you can send messages, pics, videos, links, etc. to yourself to have a self-note in WhatsApp when you don’t have any better note app.
Ironically, you can get reply from the other side here. If you change your number back, you can message from the other side.