You build the package from a source you can see and read. You decide which features you want to build in and which aren't needed. You can choose build options, optimisation and whatever else fancy stuff you want modified. With a binary distribution this simply isn't possible.

Since everything is being built on your PC you can fine-tune the code to make use of your CPU. And all it requires is two lines of string variables in a global config.

There are more than 19.000 packages available in the official repository. And even more with overlays.

The usage of advanced features like USE flags makes it more customizable than any binary distribution.

It's useful for both beginners and professionals. For the installation, Gentoo offers various types, which are referred to as stages. Basically meaning how in depth you would want to go into the process of installation. For beginners it's useful to choose for a starting distro due to its various stages that can be very time consuming but beneficial as you learn the composition in general of Linux.