The title says it all.
I am not one of them (although I wish I were) but those who’ve been DJs themselves with names somewhat known and respected in venues (local or otherwise) for 15yrs+ typically have lots of CD-R/RW written as CD-DA.
Such disks are typically obtained by the DJs from artists/producers who would give them out for promotional purposes to DJs that they trust and often contain tracks that are not intended to be released.
So, no substitution exists for the tracks found on most of such disks.
Today, except for special occasions, DJs prefer to show up at the venues with one or two USB thumb drives containing tracks for the night.
The trend is said to have started around 2009 when PioneerDJ released CDJ-2000 (which along with two later variants became the industrial standard models in venues and festivals).
Further shifting DJs away from disks to chips, additionally, are the presence of decks that no longer include CD-DA playbacks (e.g. CDJ-3000).
So, labeling and organizing thousands tracks ripped off of such disks is an essential part of the job preparation as a DJ today.
You would need lots of drives to facilitate the process of importing thousands of such old/only-to-be-heard-in-clubs tracks, wouldn’t you?