In order to connect the displays in cascading mode, the data output pin (Dout) of one is connected to data input pin (Din) of the other display. All the displays are connected to the same CS pin.
Now in order to display any character (For eg 'A'), Think of the Registers from 0x01 to 0x08 as the addresses of the 8 ROWs and we need to turn the respective LEDs ON in each ROW.
LCD 16x2 have around 16 pins which needs to be connected to microcontroller in order to make it work. But by using an I2C extender, we only needs to connect 2 pins (SDA and SCL) to the microcontroller.