The SPI bus can operate with a single master device and with one or more slave devices.
If a single slave device is used, the SS pin may be fixed to logic low if the slave permits it. Some slaves require a falling edge of the chip select signal to initiate an action. An example is the Maxim MAX1242 ADC, which starts conversion on a high→low transition. With multiple slave devices, an independent SS signal is required from the master for each slave device.
Most slave devices have tri-state outputs so their MISO signal becomes high impedance (logically disconnected) when the device is not selected. Devices without tri-state outputs cannot share SPI bus segments with other devices; only one such slave could talk to the master.