Back in the early days of personal computers DOS managed very little. One of the Operating Systems primary responsibilities was to manage your local storage (or Hard Disks). CPM, the first, and initially most successful personal operating system, had set the standard that the booting Hard Disk was the primary data source and assigned it the A: drive letter. Other drives would get assigned letters based on their availability.
QDOS (the version of DOS that Microsoft bought to produce PC-DOS and ultimately MS-DOS), chose a slightly different (and more logical) path. By reserving A: and B: for floppy drives, and starting all Hard Disks (and ultimately CD-ROMS and Network Drives) at C:, developers could write simple code to determine if a path provided by a user was a path to a removable media and required a prompt. While these conventions exist to this day in the most modern versions of Windows, they are totally absent from Apple and Unix based operating systems. When setting up DOSBox it is important to understand the C: Drive, because classic DOS software expects a certain minimum availability of drive space.