[M95D]

The main advantage of Gentoo is not building from source. One could argue that's the main disadvantage of Gentoo.

An ordinary Linux can be configured by installing pre-compiled packages. The system either has the package or not. Gentoo allows the package itself to be configured. Think of package ./configure arguments; they're abstracted as "USE flags" in Gentoo and allow tweaking the package internal configuration. It also allows modifying the environment variables used during the build. Setting CFLAGS/CXXFLAGS allows optimizing for the system's CPU instead of doing a generic x86-64 build.