I know, I'm just questioning the actual reason behind why someone would use it.
I guess that's good a reason as any.
A lot of Linux software, being open source, is published with a metric fuck ton of configuration options that are applied at compile-time, and which typically affect the software's dependencies. Binary-based distros select these configurations upstream and therefore the dependencies for you. If you want any choice here, the package maintainers normally have to maintain separate packages (for example, it's typical for emacs to be shipped as a "-nox" version if you only intend to use it on, say, a headless server).
A selling point of a source-based distribution is to give the user access to this compile time configuration, and therefore more control over the dependencies and what libraries end up on their system. Gentoo gives you a lot of customisation here. If you want to run it on a server or some shitty device, you can really pare it down, or you can harden it by removing potentially vulnerable dependencies on your software and shrinking its attack surface. Or if you want to play on the bleeding edge, you can compile less battle-tested versions of the software. Compilation times suck, but you can set up Gentoo to cross-compile from a build server, or just some faster machines you have spare.
Gentoo takes this philosophy to the kernel, whose configuration is described a bit in the installation docs. You don't have to configure the kernel yourself, but it's not uncommon for Gentoo users to do so.
I've always had a bit of an irrational minimalist ethic, so I liked how you could set flags in Gentoo to reduce your dependencies. I went years without having GTK anywhere on my system, just by setting up my USE flags to cull it (USE flags being the way that Gentoo exposes compile time configuration of userland packages).
I can't say much for the performance you get when you've compiled everything for your specific architecture. I never tried to benchmark it. And I think, on reflection, I'd have been happy to have more generic binaries compiled with my USE flags that I could have downloaded without compiling them locally.
I'm happier with NixOS. It's still effectively source-based. But it doesn't expose compile-time configuration through anything as coarse and brutish as USE flags. Instead, all package management is through a very elegant and turing complete (but side-effect free) programming language, where it's pretty easy for package maintainers to provide configuration and customisation hooks, and pretty easy for users to configure and customise. Customising a package is often as easy as adding a single "override" line in a central configuration file and reinstalling that package. Uncustomised packages are cached upstream, so you don't need to compile much.
