ESP8266 Plug is a small USB to serial bridge intended to ease debugging of ESP8266 WiFi serial transceiver. Using your computer’s USB for serial port emulation will definitely make debugging easier than doing first time discovery with microcontroller.
Even if your ESP8266 gets a bad flash and you cannot use it any more, this board can help you by either using bottom jumpers to permanently switch into bootloader mode or make a custom bootloading solution via DTR control. Design files are available on GitHub. There you will find ESP8266 command reference too (in README).