Why is the VCCIO_2 connected to the Button?

I am building the icebreaker bitsy and in the schematic there ist the VCCIO_2 connected to one side of the Button and is going to be pulled low when the button is pressed. Is this for the DFU-Bootloader or something else?
I don’t get why you would pull the VCC_IO as long as the Button is pressed low.