VBUS Pin on USB Communication
A host must provide 5V power supply to device. A host will wait for a device to appear before doing any communications to device. Host always initiates any communication.

A device can use 5V for powering itself, or if it is self-powered, simply sense when host connection is made or disconnected. A device will always wait for communication from host.

OTG device must output 5V when in host mode and not output 5V when in device mode. In device mode it can use it for sensing connection or charge batteries.