Every roof type and material require different venting methods. For steel, the steel roof vents itself. We vent the attic by installing a full-length ridge vent with metal roofing capping.
Asphalt shingles need to be vented, or the shingles can get cooked, resulting in curled shingles and causing the roof to suffer damage. Aimed installs a vented ridge cap along with surface mounted vents in areas where extra venting is needed.
Cedar and composite roofing systems require ridge venting and mounted vents.
But it is most important to understand the building first. Some roofing structures do not require venting.