This means 5 mana 10/10s and a variety of other benefits are available for these decks
A number of different anthem effects (such as Chitterspitter, sylvan anthem, etc) as well as cards that buff your whole army (Return of the Wildspeaker, Overrun, etc) can make this enormous army of 1/1 tokens into a deadly threat
Assuming youre running a lot of fetchlands and a smattering of actually good ramp, it still takes quite some time before you can play a single permanent gratis and that permanent card has to be cast instead of merely being plopped onto the battlefield, so despite everything, youre still vulnerable to Counterspell
Also, blinking Swiftwater Cliffs or Thornwood Falls adds some marginal (but sometimes relevant) value versus aggressive decks
When this Aura leaves the battlefield, that creature's controller sacrifices it