<< I am not sure how I would get such entitlement and why I should need it? >>
Apple used to charge a substantial sum for its MacOS software. That was completely appropriate, based on how many people were required to produce, maintain, and support it. Microsoft still charges a non-trivial amount of money for Windows software.
Around about MacOS X 10.8, Apple decided to reduce the charge for MacOS. But they left in place the mechanism that you "purchased" macOS, they just reduced the cost to zero. There was still the tradition that you presented your Apple-ID in good standing, that you used a Macintosh computer with a valid serial number for the download, and then the software you received was modified as it was downloaded -- tagged with the Apple-ID of the "purchaser".
Later, if you went to re-load an older, non-current version of MacOS, The Mac App Store looked for it in your Apple-ID purchase History. If you never purchased that version, it was "unavailable..."
Readers here answered questions over the years about why the got the "item not available' message, and figured out the reason I posted above.