Why do I need a wetsuit anyway?

Buoyancy.  A triathlon-specific wetsuit will provide buoyancy to your body.  In addition to the safety factor this element contributes, the suits are manufactured such that the degree of buoyancy provided to each area of your body gently forces you into an optimal swim position.  With the suit positioning you correctly and providing lift, you don't have to work nearly as hard to stay afloat as you would sans suit.  This is particularly important in a sport where you're going to need to have a lot left in you for an upcoming bike and run.

Warmth.  A bit of a no-brainer here.  A good quality wetsuit will help you maintain your body heat instead of allowing all that energy to dissipate into the surrounding water.  The suit traps a thin layer of water against your skin.  Your body heat warms the layer and it, along with the suit, provides insulation against the surrounding colder water.  In addition to reducing shock, the suit allows your body to focus more of your energy on performing instead of maintaining temperature.

Speed.  This goes back to flotation.  If you're expelling less energy staying afloat, you can put more of your energy into your stroke and propelling yourself forward.  Additionally, triathlon wetsuits are manufactured using a variety of technologies to give you grip where you need it most and some serious friction reduction where you don't.