Again , you know this , because ?
You're confused about 2 things :
-having all the info available ( something splatoon absolutly does )
-Making strategies with your friends , or reacting to your opponent strategies on the fly.
Splattoon is a game where the goal is mainly territory control, Voice chat allow for more stratégies and possibilities than without.
Basic online games where teamwork is needed require very simple things, the audio confirmation that a teammate is doing something ( without checking the map ) , the audio hints that another teammate might have an idea or might be in trouble with something. There are countless possibilities.
Even if you don't make it a default feature, even if you limit to friends , this is a potential feature that is gone from what we know.
Voice chat , even in a limited or controled state , should have been in the game. The original statement from IGN that the game won't be able to grow without it shouldn't be taken lightly.