Who would go to the Arctic with shorts and a shirt or to the Caribbean in heavy woolen clothing? Nobody would, right? So, we dress for the occasion.
Water conducts heat away from the body 20 times faster than air. Therefore, an air temperature of 25° Celsius/77° Fahrenheit means that in water with the same temperature, you will start to feel the chill in a short amount of time. Further, even with exposure protection, the length of submersion will eventually cool the body down to the extent it becomes uncomfortable. As soon as this happens, get out the water and warm up!
Overheating can also be a problem before diving. Your exposure suit should be put on after getting your dive gear ready—just before the actual dive—and taken straight off after exiting the water because a suit limits the normal emission of heat in the air. Remember, if you are not feeling comfortable for the environmental conditions that you are in, then tell your buddy/dive master/instructor and take appropriate action to either cool down or, in the case of cold, warm up.
The other reason why divers wear exposure suits is that they protect against cuts, stings, abrasion, infection, and sunburn if bare skin is exposed. Exposure suits are broken into three primary categories: skins, wetsuits, and drysuits. Each suit is designed for a specific application or environment.