Why are the compressor and turbine blades of a jet engine shaped as airfoils? Why can't they be flat?

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    $\begingroup$ Related: What is the performance of a flat plate wing? $\endgroup$ – fooot Feb 16 '17 at 22:29
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    $\begingroup$ @CarloFelicione that question is about changes along the length of fan blades, this one is about the profile of turbine and compressor blades. Different parts and different properties. $\endgroup$ – fooot Feb 16 '17 at 23:31
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    $\begingroup$ Guten Tag, Manfred - would you mind to spell your name correctly? $\endgroup$ – Peter Kämpf Feb 17 '17 at 8:56