They help you maintain a stable software stack over time. And generate a fully contained software image that can be flashed without needing the internet afterwards. (All downloads happen during compilation)
I'm pretty severely on the noob end of the scale when it comes to software, so I don't know if maintaining a new distro for myself would make sense, since all the tutorials I'm following assume I have Raspbian and all its built-ins available to me.
But maybe in the post-apocalypse, some hero with a prebuilt Rasp-yocto will rescue all our useless boards.