There’s a performative aspect involved. Similarly, it’s not a waste of time if a dance or a music jam is performed but not recorded, even if there is no audience.

An advantage to drawing something that will be erased – instead of just imagining it – is that it helps train your fine motor skills, and allows more intense cognition (i.e. you can draw something, stop and think about it, then draw some more, instead of relying entirely on visual and short-term memory.)

Finally, if you have writer’s block or a similar anxiety then a whiteboard is low-threat because a quick wipe and it’s gone.