"Different strokes for different folks." Some people come to this game from a tabletop DnD background and enjoy crafting characters and back stories before playing. (well, you already kind of do that when you choose your basic background in Character Creation don't you?) Some people come from playing the BG series in the past. Some people come to this game from playing various other RPGs and MMOs and relate it to how other games worked in the past. (that's me)
Anyway, this game allows you to ignore the origin characters if you want, and have your own custom party. You just miss out on the character interactions/conversations, and have to have them all in your head - or with your friends on Discord while playing together in multiplayer. That is fine.
I personally have enjoyed playing my own characters, getting to know the existing Companions, and am now looking forward to being able to play as one of them and see their perspective on things. I feel that will make for a richer game experience as you can "get into their heads" and figure them out more. I particularly look forward to playing as Astarion and Lae'zel - who are both my favourite companions.
Point being - I don't think there is a "wrong" or "right" way to play this game! The system is flexible enough to allow all these different ways of playing, and none of them are wrong.