It is mostly to show stories that aren't really related to the main story of the game (Barbatos and the hero, Kaeya and Diluc, e.g.) There is no definite plot whatsoever; there is in the game. I suggest you play the game if you really want to know more, it's really good.
I am playing the game (love it btw) but once I was searching up something and then it said there's a manga so I checked it out..so I wondered what the point of the manga was,,was it to tell some extra stories, or stuff that happened before the actual plot..so these are just random stories before the main plot!?
Yesyes huhu~ (⌒▽⌒)
What's the point of this Manga!!?