For a natural ventilation of any attic and roof, there must be a cool air intake and a hot air exhaust. Mother Nature takes care of the rest. No electricity needed.
With gable vents placed at the highest level of the gable wall, they are usually the best heat exhaust vents the house could have. But soffit and cornice vents are where the majority of cool air enters to push the hot out. No matter the shape of the vent, at the highest possible point nearest the roof peak will vent the most of the heated air. This simple and Natural system is why it's been used and trusted for centuries to prolong the home's life. The two most important parts of a home is the foundation and the roof. The foundation holds everything up while the roof protects it all.
Gable vents placed lower on the gable wall can get confused and alternate which vent is an intake and exhaust. At this point soffit vents are of no use and ridge vent may need to be installed. The hottest air will always rise to the peak or the highest point of an attic trying to escape.
Are ridge vents beneficial or required? In most cases, no. But if the attic run is too long, roof vents or ridge vents can add the ventilation needed. Gaps in ventilation allowing air to sit, heat and become stale is where problems occur. An attic with a 60 foot length or more will need a break in the ventilation in the middle. Air is usually good for about 20 feet naturally so a 40 foot attic with a vent on each end will suffice. Over that the air usually does not have enough force left or slows down too much to do it's job.
But, just as allowing an attic to overheat can deteriorate shingles and wood twice as fast, over cooling in the winter can cause cool moisture build up or even ice and cause mold and mildew to start and eventually rot. This is why, if you question the vents of your home, use a thermometer on the hottest day of summer and the coolest of winter to gauge more or less vents.