it's a thing, mainly the in-game communication only gives you the ability to call out things you want. In voice communication can be way more elaborate.
i.e in 10 sec's i'll be here attack this then switch here or going to faint attack switch to whm etc.
You get the point and ofcourse communication is a skill, so going on discord with people that aren't good at it doesn't really help.
But in higher skill levels it sure matters.