what is the role of Header PreCompilation as is done in MSVC using stdafx

Precompilation header purpose is mainly to reduce compilation time of huge C++ code base if used appropriatly.

Essentially, the idea is to have a suite of header file that is commonly used in the application and precompile the result so that the compiler can reload the result when compiing subsequent files.

Generally, the precompiled header is compiled when stdafx.cpp is compiled.