Why does getting a turbo make an intake and header pointless?
a turbo setup completely eliminates any form of traditional intake system and the exhaust manifold (header)
a header is simply a replacement manifold with better flow characteristics to evacuate exhaust gases quicker and more efficiently to provide more power.
an "intake" will replace the stock rubber hose and airbox with an aluminum (or plastic) pipe with an open flowing cone air filter. (there are other kinds of intakes out there, but lets not get too technical just yet)
a turbo kit replaces the exhaust manifold with a turbo manifold. they essentially serve the same purpose, in that they both allow the exhaust gases a place to go. where the turbo manifold differs is that instead of just running the exhaust gases straight out of the exhaust system, the turbo manifold forces the gases to spin a turbine wheel (turbo) that spins a compressor ("cold side" of the turbo) which is used to draw in the air. INSTEAD of the intake manifold drawing the air in, it is now being FORCED into the motor, instead of being naturally drawn in by the traditional physics of the engine. read up on the Forced Induction guide on here as well as some other sites.
that is, if you want to learn more about turbos/superchargers. look up videos on youtube of how a turbocharger works, it will SHOW better than I could explain.
so since a regular air intake and traditional header (exhaust manifold) aren't needed in a turbo setup, THAT is why they would be considered a pointless, waste of money. since if you bought those parts initially, but then decided to go turbo, you'd have to take those parts off and replace them with the turbo parts. then you're stuck trying to sell used parts.
good luck, and by all means keep asking questions. it is the only way you're going to learn anything.
