As noted by TechCrunch, Apple's decision to make the older OS X iterations available online is to make the upgrade process more convenient for owners of Macs not supported by the just-released OS X 10.9 Mavericks.
Although Mavericks boasts compatibility back to systems capable of running OS X Snow Leopard, compatibility issues still exist. Also, Apple only sells the most current version of OS X on the Mac App Store, meaning users wanting to purchase past iterations of the operating system had to purchase a copy from Apple over the phone.
For Mac owners who previously purchased and upgraded to either of the two operating systems, those downloads are still available in the Mac App Store. The vouchers will allow Leopard or Snow Leopard users to upgrade if they did not already do so.
Those interested in upgrading to Mountain Lion or Lion can purchase redemption codes through Apple's Online Store for $20 each. These codes can then be redeemed in the Mac App Store, where the actual download will take place.