When OS X 10.9 Mavericks was released in 2013, it was the end of two eras: System versions named after big cats and system updates that cost money. Since then, new versions of Mac OS have been free.
Apple has also continued to sell the last two paid versions – Mac OS X 10.7 Lion and OS X 10.8 Mountain Lion – in its online store for a couple of hundred kroner each. Previously on DVD but since some time ago as an activation code via email. But yesterday the company posted the old systems as free downloads on each support page: Lion here and Mountain Lion here.
There may not be many uses for such old versions of Mac OS, but those who have a really old Intel Mac from 2006-2008 may need Lion because Mountain Lion and later systems cannot be installed. Another reason could be running an ancient program and then you can install the system in a virtual machine.
Oddly enough, the 10.9 Mavericks are still hard to come by. There is neither to buy nor to download from the support pages, and the only way we know how to get it from Apple is to download it from the Purchased tab in the Mac App Store on an installation of 10.10 Yosemite.