A road helmet is ideal if you primarily ride on paved roads—whether that's casual weekend rides, fast group rides, criteriums, or long-distance gran fondos. These helmets are designed to be lightweight with generous ventilation to keep your head cool during sustained efforts, and they have a sleeker profile that cuts through the air more efficiently than other types.
The trade-off is that road helmets offer less coverage around the temples and the back of the head compared to mountain bike or commuter helmets. They're not designed for the crashes you'd see on trails, where you might slide or hit obstacles from different angles. If you're doing aggressive off-road riding, a mountain bike helmet with extended coverage is the safer bet.
Also, if your rides are mostly short city commutes at lower speeds, a commuter helmet with better visibility features might serve you better. Road helmets shine when speed, ventilation, and low weight matter most—so if most of your miles are on tarmac, they're hard to beat.