The bigges advantage of bodykits is the material. Most of them are made from fiberglass, frb or carbon fiber. These materials are a lot lighter than the original bumpers of the car. We must say that we are talking abour high class bodykits, like taitec or mugen. It is no shock that most of the replica bumpers weigh more than the normal bumpers. Also they often need modifications which makes them even more heavy. If we look at good quality bumpers we will see that they replace the metal frame and are therefore lighter.
Except for weight saving some bumpers have aerodynamic advantages. Again- dont look at the average ricer bumper, but there are a lot of bumpers for sale that will have some gaps where you can mount your oil coolers, brake ducts etc. They are also lower, so less air will travel underneath the car. For more information, check out this article.
Except for the fact that the bodykits ar lighter and more aerodynamic they also have their disadvantages and ironically thats also their choice of materials. Original plastic or abs bumpers are flexible and impact resistant to a certain extent. Therefore you can hit a pole at low speed or drive over a speed bump with no problems. If you do this with a fiber glass or carbon bumper you will crack them. This will take more time and patience and makes the car less easy to use.
As you see there are several advantages for having bodykits on your car. Some low profile racers love them, others hate them. In the end it is a personal choice to install it on the car, and it is one that will lead to a lot of flame wars.