Many fans of older music probably have a lot of CDs in mono. Early classical recordings, old blues discs, and jazz up until the 1950s was all recorded in mono. Even later music is available in mono mixes, notably many of The Beatles’ recordings and early discs by Bob Dylan. If you rip these discs in iTunes, they are generally ripped in stereo. iTunes’ settings suggest that you can choose to rip with channels determined “automatically,” but this never results in mono files when I rip mono CDs; I always get stereo. (I don’t mean that the music is in stereo, rather that iTunes creates stereo files where the two channels are exactly the same.)
You may want to rip in mono to save space. There’s no reason to rip a mono CD in stereo; the resulting files contain exactly the same data on two channels rather than one, and take up twice as much space.
To rip in mono, you need to choose Mono from the Channels menu in the Import Settings > Custom window. And you need to remember to make this change both before and after ripping any mono CDs you have. Note that to choose a bit rate for mono rips, you need to choose the double of that bit rate. In other words, if you choose 256 kbps, the mono files will be 128 kbps; the bit rate you choose is the stereo bit rate.
One thing to keep in mind is that if you use iTunes Match, make sure you don’t choose a bit rate below 192 kbps (which will result in 96 kbps mono files). ITunes Match won’t accept files that are less than 96 kbps, so you won’t be able to match those files.
(For those who are curious, my latest mono rips are several discs of the 17-CD set of Schubert Lieder on Record and the 9-disc RIAS Bach Cantatas Project, containing recordings of Bach cantatas by Karl Ristenpart, from 1949-1952.)