CD Towers have two modes. In the early days of BBS's, CD Towers allowed you to use multiple CD-ROM drives to store music(MOD, S3m, IT, etc.) on one drive, shareware (Windows 3.1, DOS, etc.) on another, and more store on other drives. Hard drives weren't cheap and storing all the data was much easier on towers. Most BBSs rotated the CD's so each day of the week offered new content.
More modern CD-R towers allow for mastering discs, and tools such as CloneCD were originally designed for this mode.
Both setups run on a SCSI-I or SCSI-II chain, and had their own BIOS and boot configuration. The last tower I saw in commercial format a year ago weighed around 40 lbs. and was in the area of 40 bucks used. It used a Centronix-50 to 25-pin cable and worked in DOS.
DC