What is a rare book? A book that you will not find everywhere. Neither on the platforms of lawless logisticians, nor in the normal bookshop circuit.
These books are either out of print, or published by small structures, or self-published by the artist himself who despairs of finding a publisher, or even "enriched" by the publisher himself through a separate edition.
Their common objective, as you have understood, is more passion than profitability.
It's a nod to a society where uniformity is king, combined with the pleasure of giving oneself or others unique pages.