Although the bulk of PC game sales are now no longer retail, and are made mostly through retail channels (generally Steam, but other sources, such as Amazon, GMG, GoG, Humble Store, Origin, uPlay, and Battle.net are prominent PC storefronts too), there is still a crucial segment of the PC gaming market that does purchase games at retail (specially people in countries where internet bandwidth does not allow for large downloads). Which is the reason that all major PC game releases also see a release at retail, even if said release is very limited in its run.
However, that trend may be about to change soon too- Batman: Arkham Knight, by all means a very major release, may no longer be seeing a physical release for its PC version, if insider sources are to be believed. This means that if you intend to get the game on PC, you can’t bank on a disc version- getting a digital copy is really your only choice.
It won’t make a difference to the large majority of the PC gaming population, but still, there is something to be said for the final steps being taken towards a format moving entirely away from physical game releases. It is like the end of an era.