The reason that a vehicle has multiple exhaust pipes is to increase the total volume of exhaust gases that can be carried out without excessive back pressure. This is typically true of engines with eight cylinders, so four cylinders are piped together.
You will also notice that engines with turbochargers typically have larger exhaust pipes as well, because in those cases, excessive back pressure reduces the power available from the turbocharger.
Besides, many drivers like the look of multiple exhaust pipes, so sometimes the exhaust is split at the end to give the look without actually making any difference. If the split from one to two, or two to four happens right at the back of the vehicle then it is all about the “look.” One additional point is the sound of the exhaust. An engine with different cylinders piped together can have a very different sound signature. In fact, many times engineers work at creating just the right “sound” from a car to meet customer expectations.
In addition, some of the energy produced by an engine must be used to expel the exhaust gasses. So larger or shorter or multiple exhaust pipes minimize that lost energy leaving more to be used for the application.