Contrary to popular belief, the main reason triathletes wear a wetsuit is not to protect themselves from the cold, but to increase their buoyancy.
Although thermal protection is an additional advantage, the neoprene used to make triathlon wetsuits has been designed above all to increase buoyancy, allowing you to cut times by up to 15 seconds over a distance of 100 meters. In particular, triathlon wetsuits have a differentiated thickness, greater on the legs, in order to provide a more hydrodynamic trim and ensure that your legs follow you 'in the wake' while you swim.
Other features that differentiate triathlon wetsuits from other wetsuits, such as scuba or snorkeling wetsuits, are the type of neoprene, which in triathlon wetsuits is much softer, the type of cut, suitable for movement, and ease of undressing, which is essential for saving time during T1, i.e. the transition from swimming to cycling.
A wetsuit is not essential for many races, especially in summer or in warm waters, but it is still extremely useful to ensure superior performance and to ensure high comfort while swimming. However, when the water temperature is lower, the organizers of an event may decide to make the wetsuit mandatory and not admit athletes who do not have one to the competition.
In practice, if the referee decides that the fine is optional, all athletes tend to use it even when the water temperature is comfortable to reduce times and decrease fatigue in the swimming fraction.