The best thing about it is probably well-defined partial package installs.
You can build and install only the portions of a package you want, and omit features that are normally standard but that you don't happen to need in your configuration.
The feature selection switches for determining which portions of any given package to build are called "USE flags."
A USE flag can be in one of three states: if you don't set it at all, it's in a null state whereby matching package features are built if standard and not built if extra; if you explicitly set it, matching package features are always built, even if extra; if you explicitly unset it, matching features are never built, even if standard.
Overrides on USE flags can be set on a per-package basis, allowing fine-grained control of what package features to install or omit, but the real power of USE flags rears its head in their specifiability in systemwide config files. You can declare, for instance, that you definitely, without a doubt, want to use X, and therefore, if ANY package has X features, those features should be built. This makes the USE flags sort of function as orthogonal to the package set.
Using USE flags, it's possible to make a system very, very lean. Packages which, in other repositories and package management systems, might unconditionally require certain dependencies, only require those dependencies in Gentoo if the applicable USE flags are set for the packages in question. That is to say, USE flags can be used to remove large portions of the system global dependency tree, that you'd be stuck with on any other distro.
If you know, for instance, that you don't need printer support, you can set a negative USE flag for it, and trim cups, and all sub-dependencies thereof, from the dependency tree of some other application you're trying to install. The printing features are simply not built.