The mirror match is one in which the main deck is already mostly prepared to battle, but the addition of two more discard spells can make for all the difference in these fights
They are those that enable infinite combos or are too powerful for the format
With the advent of Avatar, however, Izzets fortunes may well be on the up again
However, I think having Osgirs tap ability usable at sorcery speed means opportunities for artifact creatures to be sacrificed during your own turn and generate a few more Treasures than usual
Chatterstorm is great for similar reasons, though its limited to sorcery speed, so you wont take advantage of a spellslinging opponents turn
Green loves to cantrip from playing creatures, and Varis doesnt quite do that, but he does offer some incidental value by venturing into the dungeon and eventually popping out a 2/2 token