What about boxed software? Much of the Pro software still sold only comes boxed and on DVD, look at like Pro audio software for instance, Reason, Ableton Live, Komplete, etc...there are many Pro software apps and suites that are not featured on the App store and are not downloadable, you have to have a CD/DVD drive to install them.
I know all the Adobe apps are available from adobe.com as downloads, as well as Microsoft Office from Microsoft, but much of this software is still bought and installed via optical drive, and NOT available on the Mac App Store at all.
Also, some people still like to rip CDs to iTunes, etc, and watch the occasional DVD. I have two MacBook Airs, but I also own a SuperDrive for installing those specific software suites I have on DVD, etc. I think we still haven't completely made the transition to a "No need, Optical-Drive-less" world, yet. A few more years, but many companies are still putting out their software exclusively on DVD in boxed physical packages that you must buy at a store and install on a Disc. So, we aren't there yet.