The SID is also the System Identification for various Cellular network areas. For instance, the Washington DC area is 00013. Connecticut is 00088.
The SID is generally only a programmable feature on CDMA and TDMA phones, which can be manually programmed, and may need the SID to be reprogrammed to either prevent or allow roaming by the cell phone.
Much of the trouble experienced by customer support and Tech support comes of dealers trying to be clever about solving customer problems by reprogramming the SID.