The latest version of LCD_Library of Arduino supports both 4-bit mode and I2C mode LCDs, if I am correct.
If not, then LCD_Library is for 4-bit LCD mode and for that you need RS, RW, EN, D4, D5, D6, D7 pins of LCDs to communicate.
For I2C_LCD_Library it uses I2C communication for I2C LCDs. There are backpacks available which convert common HD44780 / compatible LCDs to I2C LCDs and I2C LCDs use only 2-wires for communication and you have to connect I2C LCDs to SCL and SDA lines of Arduino in your case.
SCL = Serial Clock
and
SDA = Serial Data
If you have shortage of controller pins in your Arduino projects and need to use an LCD then you can go with I2C LCD or if you prefer simple way to communicate then also you can prefer I2C LCD with Arduino's I2C LCD Library.