When a VFR flight plan is required

Before operating across international borders.
When operating within, into, or along routes/areas designated by the competent authority for flight-plan purposes.
When search and rescue (SAR) alerting service may need to be triggered for the flight.
At night, or when operating outside sight of the surface, per local/State requirements.
When departing from an aerodrome without ATC where local procedures require notifying a flight plan or booking-out.
Even when not mandatory, filing or notifying a flight plan is good practice for SAR alerting on any flight over remote or offshore terrain.

Source: SERA.4001 – Submission of a flight plan