What is the purpose of cable select?

Cable Select allows for the faster installation of additional or replacement drives. It does this by allowing the position on the cable itself to determine whether it acts as the Master or the Slave drive on an IDE channel. Thus when replacing a drive or adding a new one, the setting of the other drive is not an issue if it also set up for Cable Select support. Otherwise, one would need to know whether the other drive installed in the system was Master or Slave, so that the corresponding jumpers could be set.