What are the address input pins used for in ds1621?
Here's what the data sheet says: -


  SLAVE ADDRESS A control byte is the first byte received following the
  START condition from the master device. The control byte consists of a
  4-bit control code; for the DS1621, this is set as 1001 binary for
  read and write operations. The next 3 bits of the control byte are the
  device select bits (A2, A1, A0). They are used by the master device to
  select which of eight devices are to be accessed. These bits are in
  effect the 3 least significant bits of the slave address. The last bit
  of the control byte (R/W) defines the operation to be performed. When
  set to a “1” a read operation is selected, when set to a “0” a write
  operation is selected. Following the START condition the DS1621
  monitors the SDA bus checking the device type identifier being
  transmitted. Upon receiving the 1001 code and appropriate device
  select bits, the slave device outputs an acknowledge signal on the SDA
  line.


Basically, if you have several DS1621 devices, you can set the hardware address for each and exclusively read up to 8 devices. The one below is set to have address 000: -