I would like the option of seeing the cards my deck is holding when I play a test game against it in the Arena. If I know what it's holding onto I'll know if it's holding it because it's some type of CC or PC that it can't play against me because I haven't played any creatures or permanents yet or it it's holding onto it because it can't afford to play it. It it's the latter I would know to get rid of the card or somehow give the deck a quanta boost.
Basically it comes down to having more knowlege about how the deck works and being able to adjust it accordingly.
