Every time it enters or attacks, you get to return a permanent with mana value 3 or less directly to the battlefield, which can mean ramping with lands, recovering utility creatures, or grabbing key combo pieces
Sieges can be attacked and damaged by any player who is not the protector, including the controller of the battle itself
While easy combo kills like this might be frowned upon in casual formats like Commander, Nadu is powerful enough to be run in cEDH and any other viable format with ease
Of course, you know who wants this the most, and thats the Selesnya enchantments deck, an archetype we didnt have in Standard the last go-round
Potentially, so keep in mind that you may get blown out once in a blue moon