Some use cases:

- simple sketch of a user interface

- technical diagrams, like for showing how a data structure is implemented, or the (basic!) messages sent between client and server, or the architecture of your network

- working out examples, like drawing the chain of events that leads to something breaking due to a bug, or a simple example illustrating what you're saying

- very short code (3 lines good, 10 lines bad)

A whiteboard can be a tool to help you explain things. It should be used to augment what you're saying in words. It's very hard to put a diagram in someone's head using words, but much easier if you can draw on a whiteboard.

You're right, though, that things on a whiteboard should be kept very simple. Lots of code, or very large diagrams, are liable to turn into a mess.

Lists, graphs, wire frames, UI mockups, brainstorming, IP-addresses, guest WIFI login credentials, sketches of Rick and Morty ...

Basically anything you would use a notebook with pen and paper for in order to extend your working memory, but you want to share with others. Definitely not (non-pseudo) code longer than three or four lines.

Diagramming? To-do lists? Math?

doodling writing funny quotes giant reminders meetings schedules announcements

Replying to OP: because you want to share it with a group of people who are in a room with you.

Why not use a piece of paper?

A whiteboard is much easier to erase and reuse.