'Why are shader-stage input variables not writable

The GLSLangSpec.3.30 says:

Values from the previous pipeline stage are copied into input variables at the beginning of shader execution. Variables declared as in or centroid in may not be written to during shader execution.

If they are copied in and the original values are left untouched, why are input variables not writable?

opengl glsl

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'Why are shader-stage input variables not writable

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