Triathlon helmets may fulfil the same basic functions as any cycling headgear but no triathlete worth their salt can head out to train or race without a specialised helmet available to them. A triathlon bike helmet must be light, aerodynamically sound and also able to offer the protection required by riders who will be going at speed. A triathlon bike helmet is perhaps the most triathlon-friendly of the bunch, with full in-moulding construction and extra features such as washable padding and improved cooling available when you buy examples from brands like Met. For beating your personal best on a time trial run there are not many alternatives that can match triathlon helmets.
The aerodynamic nature of an triathlon aero helmet means that drag is reduced and the oncoming pressure is less of an issue as air is picked up by the design and then dumped in your wake, letting you razor your way forwards. Ventilation can be an important consideration when choosing between triathlon helmets and aero helmets are not always the best at providing this by dint of their design. That is why you might choose options from companies like Giro, where swooping styles peppered with vents are able to encourage air flow and also cut down on the overall weight.
Helmets designed for triathlons can have applications across multiple arenas. Some choose them for when they are taking on the open road while others use them indoors and there is a long history of success at the Olympics for certain brands.