No one’s fielded this yet, so I’ll give it a shot.

Portage offers maximum configurability: you can switch optional package features on and off. If a package feature is off, you don’t need to install dependencies to support it, so it makes for a slimmer system.

You can upgrade many packages even if the distribution hasn’t by copying a single small file to a new name and running two commands.

Similarly, if you’re running a new or fringe architecture (like riscv) and want to try to install a package that isn’t officially available for it, you can do it fairly simply (minor edit to a text file or additional parameter at the command line). Doesn’t always work, but it’s still easier than the configure-make-make_install dance, and the dependencies are handled for you.

Portage also supports a bunch of other fringe use cases, like pulling source straight from git and building it. And you can create simple packages by writing <10 lines of text file (well, specialized bash shell script).

On the downside, Portage is S-L-O-W. It has more complicated dependency trees to resolve than other package managers, and installs most packages by building them from source (although this isn’t a requirement).

I like it, though.