There are almost 40 different planeswalkers, many at uncommon, so you get some really powerful cards with a box of boosters
(Well, unless you're playing the excellent Fire // Ice, of course.) Aftermath, on the other hand, always gives you that wonderfully useful extra card
To transform it, you just need to pay a small amount of mana
The only exceptions are Commander, which is another beast on its own, and Limited formats such as Draft, which has a minimum deck size of 40 cards
By playing creatures that have enough size to win while also being hard for your foe to stop
Its passive ability, which puts a +1/+1 counter on a creature each time you gain life, is an easy way to combo off with cards like Walking Ballista