Top 3 Reasons to Wear a Triathlon Wetsuit

A wetsuit can provide both safety and performance enhancement a the same time.

Men's Z Force 2.0 WetZoot

#3 Speed - A good triathlon wetsuit can increase your speed, a little like how a tri bike can
make you faster when you cycle. The engineering of higher-end wetsuits creates more of this
effect as it helps your glide and can even more your swim stroke more productive.  Triathlon wetsuits are made with low density rubber which contains more air bubbles = better buoyancy = more speed!  Performance enhancement feature.

#2 Buoyancy - Wearing a triathlon wetsuit makes you more buoyant in the water.  Better buoyancy translates to an athlete expending less energy keeping his/herself near the top to the water. Keeping yourself at the top of the water means less drag and better performance  with less effort especially through kicking.  This is important for triathletes who need to conserve their lower body muscles for the bike and subsequent run.  Also I find the more you kick the faster your heart rate increases.  This will cause serious issues breathing properly for new swimmers. The thin layer of air/water in the wetsuit gives it some flotation qualities, almost like a pull buoy would. Because of this, many first-timers or people uncertain of their swim abilities feel more confident in a wetsuit especially when in choppy open water.  Safety and performance enhancement feature.

#1 Warmth -  The neoprene or rubber material traps a small layer of water close to the skin that is warmed by core body temperature and delays hypothermia in water less than 80 degrees.  Safety features.

FYI - A triathlon wetsuit is made for swimming and facilitates proper arm movement for swim stokes.  A regular wetsuit lacks the needed suit flexibility in the shoulders/arms for swimming.

Resources:

http://www.clubs.psu.edu/up/triclub/PSU%20Triathlon%20Wetsuit%20Basics.pdf