For people, like yours truly, who can’t stand to have their book series looking different.
This is something that drives me crazy about my Terry Pratchett collection, which I’ve been accruing since circa 1995. While I really, really love Paul Kidby’s work (like, really), all my older titles have the original Josh Kirby covers. Not to mention some of them are paperbacks and some of them are hardcovers, depending on how much money I happened to have (or happened to be able to con out of my parents) at the time the book was released.
Oh, and even amongst the Kidby covers, the design itself has changed over the years. So nothing looks consistent.
And then there’s Eric. Which, yes, I have the illustrated hardback of. (See also, The Last Hero.)