I'm also on the ‘this-is-strange’ wagon, as someone studying as an illustrator it seems weird. Even the small local publishers I know such as Fremantle Press have artists mail them samples of their work so that they handle the art side of a book, rather than the author.
The only exception I've so far encountered are indie creative magazines where often writers/poets and illustrators/photographers/artists will (often be encouraged to) team up to collaborate without baby-sitting from the editor, although in that case if the editor doesn't like the work, they won't print it so it's fairly easily regulated.
If you have the money, I see no problem in paying an artist to produce illustrations of your characters/world/story as you see it, but it seems quite unusual that a publisher would be foisting the art side of things off to you. For one, you'd have to have to deal with contracting issues, including possibly working out royalties for the artist (if that is how it's done, I haven't worked a job illustrating for publications yet) and allowing for the possibility of the art being used in a publication should the book go to print.
I mean, this could be a perfectly fine arrangement if this just happens to be a case of a publisher wanting to give you free reign but it's also quite unusual.
By all means, have an artist work up some concept art though if it's something you'd want to spend money to have for yourself to enjoy anyway, but be cautious.