Minwu helps your little Soul Sisters grow large enough to jam, while Aerith can find a home in Legendary Matters decks in addition to Cleric and lifegain decks
As a rule of thumb, if an effect instructs a player to look at or reveal a card before it's put into its owner's graveyard, it's unlikely to have received errata
They have low cost so that you can flood the board early and attack your opponent directly
Talking about that, the interesting part about planeswalkers is the CHOICE
Chandra, Torch of Defiance has four different loyalty abilities, one more than most MTG planeswalkers manage to bring to the table - better still, each one is very useful
To win, players must each reach 20 lore, and one of the players must be in possession of the Hexwell Crown