Theyre typically printed in sets like Shadows Over Innistrad with a strong theme of transforming cards
First off, the other three colors have their own high-impact planeswalkers (Jace, the Mind Sculptor
This card retains the same effects as its earlier editions, with the enchantment allowing players to cast spells from their hand without any mana costs, meaning that high mana cards could be played with little effect on the preservation of mana
Or maybe because they lend themselves well to the grindy, midrangey decks I love playing
(Assassins, Mercenaries, Pirates, Rogues, and Warlocks are outlaws.)
Theoretically, thats the idea behind cascade, although there are ways to break that purity, as there always are with effects in MTG