[Quoth]

These are just electronic versions of a chalkboard or wax tablet.
They are very cheap from China because there is no storage or transfer. Literally you only sketch on it and the button erases it.

It does need a battery to work, usually a CR2032 lithium coin cell. I had one for a while and gave it away, so no idea how long the battery lasts. It's an electronic novelty toy.

Useful for a deaf person to carry, or travellers in China where people might share the written language but no spoken language.

Otherwise a chalk board is more durable and environmentally friendly. In the 1960s there were cardboard tablets with black wax and a plastic sheet. You wrote/sketched with a stylus and erased by pulling down a card strip under the plastic to erase it. Some toy shops still sell them. These are just a more expensive electronic version.

Not worth it.