Apple has been moving away from physical media for some time now. Their first blatant move away from the format was the MacBook Air, introduced in 2008, which did not include an optical drive. At the time, Apple was pushing for adoption of other products without physical media, including the Time Capsule backup device and iTunes movie downloads. The recent addition of an app store for the Mac moved official software distribution away from discs, as well.
By eliminating the materials, packaging, and shipping costs associated with software distribution, Apple has streamlined and simplified purchases and upgrades. They also, in true Apple style, secure greater control over what they distribute. If Apple updates their distribution files, there is no way to install an older version from discs that shipped with a computer, for instance. If Apple changes software features or functionality, there is no way for the customer to “roll back” to a version shipped on disc.
By only ever providing the latest version of any available software, Apple promotes faster adoption of newer software; customers cannot easily remain on older software that Apple no longer wishes to support. By reducing the cost of their next operating system to $20, Apple makes it very enticing for users of their current software to quickly jump on board with the update. If prices were to be reduced to $0, Apple could virtually guarantee rapid adoption of the new software, considering most home users do not skeptically wait for bug fixes and maintenance releases like large corporate installations have to.