The LCD backpacks serve to solve the main problem with using LCD which is the large number of pins required to control the LCD. This is not only annoying but also occupy many useful pins of your Arduino or microcontroller that might be saved for another application.  

This LCD backpacks by Adafruit reduce the number of pins needed to connect to an LCD. By using simple i2c and SPI input/output expanders we have reduced the number of pins (only 2 pins are needed for i2c) while still making it easy to interface with the LCD. 

T This backpack will work with any 'standard' character LCD, from 8x1 to 20x4 sizes! As long as they have a 16-pin single-line connection header at the top. 

Specifications

• Works with 16x2 and 20x4 Character LCDs (basically any LCD that have 16-pin single-line connection header at the top).
• Can use I2C or SPI connection
• Control over characters display and backlight status