In many ways, your engine can be considered to be a big air pump. In order to work properly, it is important that the system be closed to prevent leaks and unregulated air from entering. A tightly sealed intake manifold is crucial. The intake manifold gasket is responsible for the seal where the manifold meets the cylinder head, so whenever you have the manifold separated, you should always replace the intake manifold gasket. It is cheap insurance to avoid rough running caused by air leaks after your throttle body.