Many people are transferring their websites to concrete5 from other platforms, or from static html websites. In most cases the page addresses on the old website and the new one will differ.
Example #2 (let's assume that is a "Books" section in online store):
Example #3 (let's assume you have multiple files and you want to keep control over them via cms, but the download links are already spread over the web)
The problems solved by setting up 301 redirect (so-called "Permanent redirect"):
Please note that by default concrete5 allows to set 'Additional path' for a page via page properties, but this will work only for pages like an example #1 and not for an example #2 or example #3. So, if your website consisted of just few .html pages, you don't need this addon, don't waste your money.
Important: Pretty URLs must be enabled in the dashboard settings.