SD Card Reader requirement?

Gateway Quick Start list among HW requirements:
“16 GB micro-SD card and a micro-SD card reader”

Why would I need card reader? The RPi 3B has a card slot…

Just guessing:
At the time I will run Etcher in order to burn the image on the SD card the RPi will probably not show up at all anywhere since there is no image on the card…
So is the microSD Card Reader should be connected to PC and card inserted into it and will show up as additional drive?

Your guess is correct. The reader is only to put the image onto the SD card in the first place.