[vidarh]

ISBN's provide a way for people - including bookstores and libraries to index your book data. Which means if you use Ingram Spark, or Amazon's extended distribution, or any other major distributor, your book will pop up on huge amounts of other bookstores inventory systems pretty much automatically.

It won't mean a lot for sales unless you also encourage places (stores, libraries) to stock them, but it does have a small effect (caveat: self-published authors are seen as less than dirt by a lot of bookstores, as a subset of self-published authors takes the "encourage places" as "relentlessly badger places" to try to get them to stock unsellable books)

I've had a handful of sales that way, and financially it's been irrelevant, but it is a little boost to see my books pop up more places.