You will take less damage as she cannot hit both the vessel and you with q/w
she loses her sustain if you can avoid both it and her q, I'm not saying you have to avoid every one, but remember they're slow and easily sidestepped if you just watch for the indicator. of course sometimes damage is unavoidable if you want farm etc ( but you have to give it up sometimes to deny illaoi her strengths.)
and yes, one less tentacle.
but one less tentacle does make an enormous difference when actually fighting her. as that's denying
Think of it denying half her ultimates strength when you fight her in lane one extra instead of 2-3 depending on many variables,
Sorry if this is hard to follow.
Off-topic: aren't the zilean main from another post?