Imagine for example in Mission 4 (this is coming from my ass, so no spoilers) you gain access to your Super, or your Sparrow, or the Social Space, or whatever it is that you didn't have in Missions 1 - 3. Someone who hasn't gotten that far yet invites you into their fireteam to help them play through campaign for their first time and you show up with access to those mechanics which haven't been earned/revealed to the other player yet in their instance of the story. That's why after the story, you can replay it with friends. Because everyone will already have everything unlocked from the story missions so it doesn't matter anymore at that point, but initially stuff like that does matter.
I really don't get the "lol, the Destiny franchise has had one game that didn't have a deep campaign and so that automatically means the whole franchise from here on out can never have a relevant story presence" mentality.