Do I need to fit a roof fan?

Not necessarily.

An empty cabinet doesn’t need a fan simply because a fan position exists, and a cabinet containing patch panels, blanking plates and a modest amount of low-power equipment may be perfectly comfortable with passive ventilation.

The fan position is an option, not a ceremonial hole that has to be filled.

Whether active extraction is worthwhile depends on the amount of heat being produced, the ambient temperature and the airflow requirements of the equipment inside.