Any kind of reference, recipe, directions, notes. This would even be cool instead of a second monitor. Just to put up a reference page or cheat sheet.
IMO this is still less functional than an ordinary sheet of paper, this just solve the energy issue, but not much else.
I'd still probably go with a powered one just to get the benefit of swapping pages and taking notes on it.
On the other hand, if the can manage to make something that can power them over a longer distance than typical NFC, I could see those used for in-office digital signage (meeting room status and schedule, employee schedule, etc) which doesn't require a battery swap, PoE or some other kind of way to power it.
Jokes aside, you'd probably want that 480x800 (portrait rather than landscape). Which would be 80 columns and 100 rows in old 8-bit microcomputer terms. So it wouldn't be the more detailed of a document but it should be detailed enough for a cheat sheet or narrowly focused reference page. You probably wouldn't want much more than that anyway otherwise you risk your reference material to be too verbose for quick sanity checks.
Anyway, most e-readers are in the ballpark of 800x600; and I expect if the concept got traction, there's no reason it couldn't be scaled up to 8.5" x 11" at equivalent pixel densities (at the cost of hovering your phone for a few seconds longer or whatever).
A Kindle Paperwhite is 1072x1448, 300 PPI, 16 levels of grayscale.
I personally couldn't tolerate reading on a kindle until they reached 212 PPI. 800x600 may be acceptable for a simple sign, but it's not gonna wow anyone.
[0]: https://medium.com/@richardeng/syntax-on-a-post-card-cb6d85f...
Might pay to check yours and read the health effects section here https://en.wikipedia.org/wiki/Bisphenol_A
Edit: fixed a word
(edit) There is something telling about how three people have now downvoted my comment for not doing the work of pasting the links myself, but not a single person has bothered to provide the links themselves, which kind of demonstrates the problem with these comments: the comment I am responding to is undermining a well-researched notion--one that AFAIK no one questions, and for which it is trivial to find numerous articles and studies: that BPA is absorbed through the skin from receipts--with the moral equivalent of "citation needed"; that comment asking for a reference seriously took longer to type than finding the relevant articles would have, and yet in practice is asking other people to do that work, and so the work doesn't get done by anyone... but the comment itself sits there, making people who are less informed on the topic question the validity, as in "I dunno, this comment claims they hadn't heard that, and is demanding citations; if it were easy to find a citation they wouldn't be asking that, so I guess the thing they are poking at isn't actually true". If you want to downvote my response to that behavior--which is to point out that a Google search would have worked--but you aren't willing to actually do the work of providing the links yourself (or at least also downvoting the comment), you are just making yourself part of the problem of incentivizing leaving these misinformative comments :/.
[0] “I make it something of a policy to not look at things people say in response until at least a month later.”
Commenting on commenting on commenting on downvotes, which you're doing, is annoying. I'm on an even number here, so I should be okay. We'll see!
Too bad. The original "invisible ink" activated by heat is lemon juice.
I will gladly use this e-Paper display instead in any situation where I want some kind of reference sheet like the parent commenter said, and where the resolution is sufficient.