I would say one of the biggest selling points for me are the use flags: In Gentoo when you compile a program through your package manager (the package manager for Gentoo is emerge and emerge doesn't just down load a package it literally pulls and compiles the package on your system. Which is nice because that means you don't just have a random binary you actually have access to the source code and can edit set code any way you see fit.) emerge; when you download a package with emerge you can set these things called "use flags" that tell the program to compile with or with out specific things.
So, let's say you want a kde desktop environment. Normally, if you downloaded kde on a prepackage, bloated, one size fits all, noob friendly, distros like you would have no choice but to run systemd and pulse audio, but, the good thing about use flags in Gentoo are they allow you to pick and choose how your packages are compiled. That alone is a major plus for me.
Not to mention that because your package is unique to your build your binary is unique, mean, that any vulnerabilities that pop up for, say, kde wouldn't really effect your kde because your kde is unique; firefox or what ever. You catch my drift.
Also, there's the massive amount of information nd knowledge you gain from just installing and building and compiling this distro yourself that you could, otherwise, not have. You learn a lot from building this distro. I would highly suggest it. It's hard, but, it's worth it.