There are two basic reasons for replacing the stock manifold with a branch:
To replace a broken, cracked or warped stock manifold with a factory-supplied original is too costly and a branch is about half the price or even less, fitted. In South Africa, approximately 70% of all branch manifolds are sold as replacement items. Apparently, this figure increases to approximately 90% in Australia where most “extractors” as they are called are cheap press bent items as described above. (Performance doesn’t enter the picture.
The second reason is a performance enhancement. As South Africans, we are privileged to have available of the shelf designed and developed branches, which actually work well for most popular cars. These branches, matched to through-flow systems do result in significant power gains, which can be verified on dynamometers. If you are going to replace your car’s broken original manifold with a good branch, you score a bonus in terms of power gains, especially if you later decide to replace the stock silencer system with a matching performance silencer system as well.