Thank you for this. So when would I need to use the flash button on this board? 

I'm not certain, but I would think that you would only need to use it if you are programming it using a USB-to-Serial adapter instead of a USB connection directly.  The USB-to-Serial connects to the Rx and Tx pins of the ESP 8266 chip.  

At least this is my understanding.  I've never used the NodeMCU.   I just use the ESP 8266-o1 module directly.    So I have to use the USB-to-Serial method since there is no USB port on the o1 module.

To enable ESP8266 firmware flashing GPIO0 pin must be pulled low before the device is reset. Conversely, for a normal boot, GPIO0 must be pulled high or floating. If you have a NodeMCU dev kit(ESP 12) then you don't need to do anything, as the USB connection can pull GPIO0 low automatically while flashing.

I've been using just the ESP 8266-01 module for my projects.  I need to ground the GPIO0 pin to program it.   I write my own firmware using the Arduino IDE.   However, I'm also programming it using a serial connection.

According to the above information if you are connected to your  NodeMCU using a USB connected to your computer than this is taken care of for you so you shouldn't need to worry about having to ground the GPIO pin.