The exhaust manifold is the first part of your vehicles exhaust system. It is connected to your vehicles engine and collects your engines emissions. The exhaust manifold receives the air/fuel mixture from the multiple cylinders in your vehicles engine. It collects the fuel/air mixture from each cylinder, whether you have four, six, or eight cylinders. Not only does the exhaust manifold receive all of the burnt engine gases, but also it completely burns any unused or incomplete burnt gases using its very high temperature.
The manifold also houses the first oxygen sensor in your exhaust system to inspect the amount of oxygen entering the system. The oxygen sensor monitors the amount of oxygen and will tell the fuel injection system to increase or decrease the amount of oxygen used in the fuel/air mixture used to power the engine. Basically, the exhaust manifold acts as a funnel and is used to collect all of the engines emissions (from however many cylinders your vehicle has). Then once they are in one place and completely burnt, the manifold sends the emissions into the rest of the exhaust system.
An exhaust manifold is a key component of any car you see on the roads today, and it is true that you can't have a car operating with an internal combustion engine without it. The reason they are so critical is because of the emission norms we have in place. These essentially help us maintain a cap on the air pollution caused by harmful exhaust gases that are produced as a result of the way the engine produces power.