The submission of a Flight Plan is usually a good practice even if you are flying VFR in G airspace. One of the most obvious reasons is that you might benefit from a faster SAR response in case of an emergency arising.
According to the ICAO Annex 2 (Rules of the Air); a flight plan shall be submitted prior to operating:
♦ a) any flight or portion thereof to be provided with air traffic control service;
♦ b) any IFR flight within advisory airspace;
♦ c) any flight within or into designated areas, or along designated routes, when so required by the appropriate ATS authority to facilitate the provision of flight information, alerting and search and rescue services;
♦ d) any flight within or into designated areas, or along designated routes, when so required by the appropriate ATS authority to facilitate coordination with appropriate military units or with air traffic services units in adjacent States in order to avoid the possible need for interception for the purpose of identification;
♦ e) any flight across international borders.
A detailed description of the FPL can be found in the ICAO DOC 4444 (Air Traffic Management).
Finally, make sure your flight plan is closed once you land, so that nobody is looking for you!