The file speeds up compiling process since the scriptpch has common headers and it is precompiled to a form that is easier to be used when compiling other files.
Without the file compiling takes forever and the size of the files and probably required ram etc for compiling is a lot higher.
Previously on mangos the PCH file was not used by compiling and a single cpp file compiled to obj took almost 20mb. After using PCH the compiled PCH file was 20mb, but all other files were only 1-200
kb. Considering that about 20mb of information was
preprocessed and was not needed to be processed for probably hundreds of times (once for each cpp compiled), the compilation process was sped up a lot.
In computer programming, a precompiled header is a (C or C++) header file that is compiled into an intermediate form that is faster to process for the compiler. Usage of precompiled headers may significantly reduce compilation time, especially when applied to large header files, header files that include many other header files, or header files that are included in many translation units.
-wikipedia
PS. I dare you to uncheck the scriptPCH and corePCH in cmake!