Legion is supposed to be the culmination of a 23 year long story. We are finally confronting the Burning Legion for the last time. The problem? All of the action is happening on some godforsaken island in the middle of nowhere. Yes, Blizzard tried to play up the important of Broken Isles, e.g. Suramar is an ancient elven city only second to Zin-Azshari in grandeur, Val'sharah is the birthplace of druidism, Highmountain houses Neltharion's lair, Stormheim is more or less the center of titanforged presence on Azeroth, Azsuna is kinda... meh? Broken Shore is implied to be one of the most important locations on Azeroth and even N'Zoth and Y'shaarj fought for control over it many years ago.
Up until Gamescon 2015 however Broken Isles were just a chain of islands with a couple of elven ruins. None of the new lore had any build up. If we look at Cataclysm (which had a similar theme to Legion), all of the zones had some kind of build up. People had been speculating about Uldum since day 1 of WoW, Hyjal was also an established location with important lore, and so was Grim Batol and by extension Twilight Highlands. Vash'jir didn't have any build up but then again it was one of the most controversial zones in WoW's history. The Elemental Planes felt epic for obvious reasons.
I think Legion would feel FAR better if all of the action took place in previously established locations or at the very least had more Legion-controlled zones. Why Blizzard didn't hint at Stormheim and Odyn during WotLK is beyond me and the same goes for all other zones as well, they had years to come up with some lore for them.
TL;DR: Broken Isles are important because... reasons? Doesn't seem convincing.