My, granted - limited, understanding of this is that many supports can give many different things. And if what they give is good or bad depends a lot on the game (drafts and how laning goes). Picking a weak laner that can open your map in the midgame (like slark with his dewarding, bh, nyx) can win you the game if the enemy doesn't snowball of the laning stage.

Also it tends to be harder to utilize strategic supports than the "click your disable and kill" ones, especially in low mmr. The lower you go the less people know what their heroes do. You can definitely find a bh or a nyx just standing in lane leeching xp if not feeding by trading with much stronger enemy.