I wud answer from the programmers perspective, I hope u meant Tape drives by saying 'Tape'. A Virtual Tape drive emulates the physical tape drive. This was introduced to overcome the shortage of physical tape drives as they were costly.
If a job accesses a huge number of tapes silmultaneously, u wud have coded the JCL to have DD DSN=TAPE,UNIT=AFF=DDNAME to retain the Drive used by a previous DDNAME. But now after the Virtual tape u wud have sufficient drives and u dont have to use the same drives again and again. Ofcourse the Physical tape drive is faster, but that is insignificant compared to it ROI. For a detailed architecture visit http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/FRAMESET/EZ3CAH00/CCONTENTS?DT=19990412142331