The main use of it is for things like language servers. In order to properly show things like compiler errors in your editor, your language server (or linting e

The main use of it is for things like language servers. In order to properly show things like compiler errors in your editor, your language server (or linting engine or whatever) needs to know EXACTLY the command you used to compile. The most obvious thing is preprocessor macros, but also things like which C/C++ standard are you using, have you turned off some number of warnings, etc. compile_commands.json is a way to solve that problem in a "build system independent" way. It's just a list of exactly what commands where used for what files. As long as you generate that file correctly, you can use whatever build system you want (cmake, hand-written make file, 1000s of lines of Perl 5, a shell script from 1983, whatever) and your editor can correctly show errors, warnings, provide auto-completion, all that good stuff. An alternative (at least for clangd) is to use compile_flags.txt, where you can just specify whatever flags you want the compiler to use EDIT: if you're asking why this person in particular wants compile_commands.json files, it seems like he just wants a unified format for the compile commands of all the files in all the packages so he can use it for his analysis or whatever.