As a game collector, I own quite a few games for multiple platforms. I.e. “Ecco The Dolphin” for Dreamcast (“primary platform” for this game) and PS2 (“additional platform”). But it is only shown in the Dreamcast-list, not the PS2-list. So when opening my PS2-Games list, i actually don’t get my full games list for that platform!
I guess this filtering makes sense for other ways of sorting, but not in this specific case.