This is a very important question to ask since the two might appear the same. But deeper evaluation would reveal that there is something special about this bike that could not be seen on a regular road bike.
But before enumerating these, you must remember that using such dedicated bike is not limited to triathlon events. Even on solo ride, it will ride just as great.
This bike provides an easy transition from swim to bike and from bike to run. And this is perhaps the most special thing about triathlon bike. Remember that transition from discipline to another is still part of the race and therefore must be done smoothly. Since bike comes after the swim and before the run, the bike used should be designed in a way that the triathlete can easily mount and dismount.
Among all 3 events, the longest part of the race is bike and therefore, the bike to be used must be designed in a way that it can provide comfort to the rider. It is designed in this way. Also, the design is very efficient in giving the aerodynamics that provides speed. The elbow rest are designed to be low so that the rider maximizes his or her every kick. The Garside Study in 2001 disclosed that triathletes who use them run faster that those who use the regular road bike on the same covered distance.
From bike to run, the triathete should have enough energy to finish the race. Since it is designed to be efficient and comfortable, the rider saves a lot of energy that is important to finish the race. To be particular, riding it provides less cramp on the lower back muscle due to its geometrical construction. It is important to note that a road bike can cause your muscle to tighten that might effect your transition especially on the first two kilometers of the run. This would give you enough difficulty to keep you from effectively maximizing the time.
To simply put it, the triathlon bike would give you the speed you need not only on the bike but also on the run. These advantages provide you with the much needed energy and comfort during the race.