It provides the ability to grind out when you run out of cards, almost giving you an extra card draw each turn, but better, as it's a permanent of your choice from your graveyard, nulling any efforts by your opponent when using removal spells
In a way, the legality of gold-bordered cards is quite similar to proxies
It was ubiquitous in all formats in any deck that could run it
Because you cast whatever creatures Azula swipes, you don't need to worry about giving it back if your opponent removes Azula like you would with Mind Control variants, and the enters ability means your opponents are always two-for-one'd at best
In addition to these bans, which we'll get to in a moment, Wizards has stated they are going through a review of all cards that have been printed and will be taking action on similar cards in the future