Well, name of the package should be a hint. Xorg needs that specific package to be able to run (
x11-drivers/xf86-video-amdgpu). That package is for Xorg to interface correctly with
amdgpu.ko
driver that is installed by
graphics/drm-kmod. There are a few more
x11-drivers/xf86-video-NNN
drivers in existence, they were written at a time when GPU drivers were simpler and less diverse. If you run Wayland, you don't need the
x11-drivers/xf86-video-NNN
drivers. If you run Xorg, you do, and the correct one depends on the GPU you have.