That’s for the factory to program the initial firmware. There is no built-in USB serial chip on the controller so the initial firmware needs to be programmed in through that header. After that, subsequent firmware updates can be done over the air.
If you have a serial adapter yourself you can obviously solder pins onto the connector in order to check serial output etc. The pins are clearly marked (TX, 5V, GPIO0 on the front side and RST, GND, RX on the back side). GPIO0 and RST are optional — you don’t need to connect them, they are meant to trigger the firmware upload mode, and their usage is compatible with nodemcu.