I am using cmake to setup my project and when I change a file in a project, I found my cmake knows to only recompile the changed file and then relink everything together for the final executable/lib.
I then read through the documentation about ccache, what I don't understand is: what is the difference between ccache's approach (that uses hash value to check if file is changed and needs recompile) and the default approach that the cmake uses (or there might be something else rather than cmake checks the file updates, but you know what I mean here). Maybe the PCH part is different, but cmake 3.18 now comes with PCH support, so, does that mean the benefits ccache provides on the PCH part is no longer unique?
Consider the case where you switch to some older branch of your project - that you did compile in the past and that ccache has cached, but CMake sees as "almost all files have changed and must be recompiled" - that's where you see a massive gain.
Another situation is where you have deleted your build directory (for some good reason) and now have to rebuild everything. ccache is also a huge help there.
Also; ccache is trivial to set up and is thenceforth completely invisible / transparent, so there really is no reason to not use it. When it helps it usually helps a lot, when it does not help it doesn't hurt.