This board offers a novel way to control power to any Arduino-based boards. Pressing the button latches power to the board, and the microprocessor can decide exactly when to remove power to itself. This can be helpful when developing solutions for low power applications. A remote doorbell perhaps.
Essentially, pressing the button turns on Q1 applying power (+5V) to a host arduino / ESP8266. A GPIO from the host processor is set to "1" and that GPIO continues to hold Q1 "on". Once the host has finished it's task, it sets the GPIO to "0" which turns off Q1 providing SW1 is no longer depressed.
This board is similar to the schematic Rui Santos made for his Latching Power Circuit. I added a few embellishments, a power-on indicator, a mini USB connector for power, and did some BOM consolidation.