Who this suits
People who deliberately chose not to buy into the biggest e-reader ecosystem: readers who borrow from libraries, who want a more open file format, or who simply prefer a different device.
If you know somebody made that choice on purpose, a card here lands far better than one for the obvious alternative — and if they did not, it is the wrong gift entirely, because the balance does not cross ecosystems.
Check which e-reader they actually own.
E-books and audiobooks
The store sells both, and the balance covers either. Where audiobooks are sold on a credit-style subscription in a market, that is a separate product from balance — same distinction as Audible.
Library borrowing
Devices in this ecosystem commonly integrate with public library lending. That does not cost money, so it is not what the balance is for — but it is worth mentioning to a recipient who does not know the feature exists, because it changes how much they need to spend.
Telling somebody their device borrows library books free is a better gift than the card, and costs nothing.
Region
Store and catalogue are country-specific, and rights differences mean titles vary by market. Confirm the country.
Amount
The price of two or three books reads as a proper gift. One book is a token, which is fine if that is the intent.
Asking for one
Country and amount. See the books and audio guide for how this compares with the alternatives.