Under current rules, if you want to profit from the sale of your work, you must have an ISBN. This is also required to allow your book to be distributed outside Bubok and entered into the commercial book market. A unique ISBN can also be used to distinguish the different versions of your book so that customers can always be sure that they are buying the right edition.
ISBNs are not for serials publications (magazines, newsletters, etc.). If you are publishing a product with these characteristics (for example, a monthly literary newsletter), consider getting an ISSN (International Standard Serial Number). ISSNs are for print, electronic or web material that is produced continuously and serial. Your title should use the same name with the exception of the number of volume.
If you want to sell your book at cost or for free download then it is not necessary that you have an ISBN.