If you are publishing your own book, you need a barcode on the back cover. The barcode, based on your book's ISBN, goes into a database and will identify the item when merchants scan it. It does not encode a price. It's simply an identifier used for inventory. If you want merchants to stock and sell your book you need a barcode.
I had paid for a barcode to be placed on the back cover of my current project, The Woman Who Values Herself, and when I got the final cover PDF it occured to me to print it and test it with a barcode reader app. It wouldn't work, although the app read other barcode labels. I kept trying, freaking out incrementally. Because the book is so small, the barcode had been shrunk proportionally and it was too small for the app to read. Online I found that there is indeed a minimum size for barcodes: 80 percent of the original, or about .825" high.
Having advised the cover designer of these facts I was in turn advised that she'd never had any problems with shrunken barcodes, but she'd enlarge it just for me, and so it was on the next proof. The barcode scanner could not read this barcode either. Feigning great patience (THIS BOOK HAS BEEN IN PRODUCTION SINCE JUNE for PETE'sSAKE!!) I advised her of this and asked her to test it on her end.
The project manager contacted me and swore it worked on their end, and it wasn't working for me because my proofs were electronic PDFs and low-resolution (high-resolution PDF proofs are so huge they'd crash a mailbox) although they didn't look it. So I chose to just drop the issue, now that I had his email saying it worked--in case it didn't. So ended this tiny nightmare, and I learned:
1. You need an ISBN and a matching barcode.
2. Test the barcode.
3. There is a minimum size for barcodes, and even if it is plug-ugly and out of proportion to the book's size or design, you still need one if you want stores to carry the book, and of course you do.
4. Understand that your electronic proofs are low-resolution.
5. Get written assurance that the darned thing really works so that if it doesn't, this can all be done over again at somebody else's expense.
6. Everything in publishing works far more slowly than you'd think.