355 engine compartment tubing

It is to strenghten the subframe. It is a triangulated member.

 

If you are not talking about square section tubing which is structural as explained by Diablo then you're probably referring to the secondary air injection tubes.

They take air from the secondary air pump (right hand side rear corner) and feed into the manifold up-stream of the catalyst. Purpose is to inject air into the exhaust stream which causes an "afterburn" of the unburnt fuel. This heats the catalysts to operating temperature very quickly.

Catalysts only start to convert gasses when they reach 450DEG C. To ensure afterburn the car uses a CSSR (Cold Start Spark Retard) strategy retarding the spark timing to ensure plenty of unburnt fuel reaches the ex-manifold. This is just another reason why it's never a good idea to thrash your engine until it has warmed up.

Rob.