What's the point of a USB to Serial converter in Arduino Uno?
The problem with your understanding is with what "COM" is.

"COMx" is not a physical serial port - it is merely a concept. It is a "serial device" in Windows.

That Serial Device is implemented as a CDC/ACM protocol on USB. It's not "converted" - "COMx" is just the visible interface to the PC's end of the CDC/ACM connection.

The ATMega328P chip (and the ATMega2560) can't talk USB directly. They talk UART. So UART data is sent to another chip that can talk both USB and UART to bridge the two technologies.

So from the PC perspective the chain is:

CDC/ACM Driver ("COMx") =&gt; USB =&gt; ATMega16U2 =&gt; UART =&gt; ATMega328P


Boards with an ATMega32U4 chip on them, like the Leonardo, can talk USB directly - so the sequence is much shorter:

CDC/ACM Driver ("COMx") =&gt; USB -&gt; ATMega32U4


Again there is no conversion from "COMx" to "USB" since "COM" is just the name given to the exposed end of the CDC/ACM driver. 

It can be said that CDC/ACM emulates a serial port over USB. It forms a channel between the CDC/ACM driver ("COMx") and a pair of bulk endpoints in the chip at the other end of the connection.