(I posted at the same time as chrisknightley, but I'll leave it)
That's incorrect. One is the ATmega328P and the other is the ATmega16U2. These are two different microcontrollers.
The ATmega328P is the primary microcontroller your sketch runs on, so I'd say this one is essential.
The ATmega16U2 provides a bridge between the ATmega328P and the computer. This chip communicates with the computer via a USB CDC serial port and with the ATmega328P via UART. The ATmega328P doesn't have any native USB capabilities, so it can not easily communicate directly with your computer. If you want to upload sketches and use the Serial Monitor, you'll need some similar chip, but there are other alternatives such as FTDI FT323R and CH340.