Plus, you can remove the Post-Its when the errata changes, or if you decide you want to get rid of it later. Way better than writing it directly into the book or using tape.
In the legal publishing area (law books, state statutes etc...) a hardbound book may only be published once every five years or so sometimes longer. In the meantime a "small" booklet will be published with changes and updates to the book that is stored in a pocket in the back cover of the book, these booklets are published anywhere from monthly to yearly depending on the book.