The one fewer mana makes a difference when you’re trying to combo before turn 10, and the Roar sometimes gives you more options, like the ability to combo with a single Naturalize (if your opponent has a minion to trade your Anaconda). But you’re correct – not strictly necessary for the combo. If you’ve got Anaconda, Mecha’thun, Naturalize, and some way to kill the Anaconda in hand, you can feel free to burn your deck.
The one fewer mana makes a difference when you’re trying to combo before turn 10, and the Roar sometimes gives you more options, like the ability to combo with a single Naturalize (if your opponent has a minion to trade your Anaconda). But you’re correct – not strictly necessary for the combo. If you’ve got Anaconda, Mecha’thun, Naturalize, and some way to kill the Anaconda in hand, you can feel free to burn your deck.