Function of 16u2 microcontroller

I'm new to microcontroller programming. I've been wanting to buy an arduino UNO R3 and it was specified that the R3 uses a 16u2 instead of the 8u2 used on the previous generations.I don't really know what's the application of 16u2 or 8u2.I checked the atmel website but couldn't really understand its function on the arduino board.Also the freeduino versions don't seem to use it.So I'm really confused and would like to understand its application and alternatives.Could someone clarify this or point to the resources where I could read.Any help is appreciated.
Thanks and regards.

On the Uno, The ATmega16U2 (and 8U2) provides USB-to-TTL-serial conversion.

The same service was provided by a chip from FTDI with previous Arduino boards and very likely with the Freeduino.

So is there any advantage of having these over the ftdi chip used on the older ones and other compatible boards?

Yes. They can be reprogrammed so the Uno is presented to the computer as something other than a serial port like a keyboard or mouse.

Or midi device...