[Tx Firefighter]

Because the intake manifold hangs off the side of the engine and doesn't get much heat from it. The intake manifold runners need to be somewhat warm to keep the fuel atomized well for decent drivability. Hence the lines to use warm coolant into the manifold.

The original manifolds didn't need this since they were married to the exhaust manifold and the heat conducted up into the intake this way. Your aftermarket aluminum manifold won't have that option.