I use it as a scripting language for my game engine, but also for all the tooling around it. Premake for generating the VS solution/makefiles, but also my own tools using IUPLua [1] and a few other modules (lfs, sqlite etc). Pretty much my whole ecosystem is C++/Lua.
[1] - https://www.tecgraf.puc-rio.br/iup/en/iuplua.html