What is the point of this character? For what purpose does it exist?
I think, story-wise, you're very much intended to be a blank slate. You were abducted by mind flayers and implanted with a parasite. *This* is what makes you special. I mean, think about it: one of the first encounters that you can have is with a group of fishermen that have been mind-controlled. If you try this encounter without companions you'll *probably* get your ass handed to you. You're basically a barely-competent adventurer. For that matter: the opening cinematic features the mind flayer ship just snagging every Tom, Dick, and Harry it can off of the street of Baldur's Gate. As far as we know thus far: there's *nothing* special about you, you were just thrust into this situation and have to make the best of it.
That being said: your character doesn't lack distinguishing features and they're not completely "blank." What I've noticed in my multiple playthroughs is that the selections you make at the start of the game are actually fairly impactful RP-wise, much more than I expected them to be. If you're playing as a Githyanki you'll find several skill checks that you simply bypass because your whole species has dedicated themselves to hunting mind flayers, so naturally you know quite a bit about them. If you're a rogue you'll immediately comprehend what's happening when you encounter someone trying to swindle you and be able to call them out on it. Your background, class, and selected skills *do* have an impact to an extent that I certainly didn't anticipate coming into the game.
I will concede that I have the added benefit that as an adult I *have* played a significant amount of D&D now; when I was making my initial character I was able to fully comprehend what I was doing. I *did* play Descent into Avernus, so I immediately recognized the context when places like Elturel and Avernus were mentioned (although the game's canon and our campaign's canon don't appear to have played out the same way). I'll also admit that I absolutely have a habit of creating my own head-canon for characters I make in *any* RPG I play. That means the very same thing that you're complaining about is actually kind of nice for me - I'm not shoe-horned into some backstory that conflicts with the narrative I've got going in my brain. As someone who, during character creation, envisioned my BG3 character as a D&D character that I would want to play I felt *extremely* rewarded when I realized over several runs just how many dialogue options that stuff opens up and I appreciated not being stuffed into some generic backstory that might conflict with the decisions I want to make.
I also want to point out that when I played Divinity 2 I felt like I was missing out by not playing one of the origin characters. You would approach an NPC, they'd have a discussion, you'd move on to the next NPC. As a custom character I had no particularly special storyline and I was basically just opting out of having my own character-specific questline. While I think that the origin characters in BG3 are cool and all, I don't feel put off when it comes to creating a custom character. They're verbose enough with what they're going through and what they need (assuming you take the time to talk to them) that I don't feel as if I *have* to play one to experience their story. Obviously we'll have to see how this plays out with the rest of the game, but as of right now I'm planning to make a custom character for my first playthrough on the full launch which was absolutely *not* the case with Divinity 2 for me.
So I guess the TL;DR is agree to disagree. Obviously I would *very* much like to see the voice lines put in for custom characters because right now I feel like a weirdo that just stares and smiles at people instead of actually *speaking*, but as far as character origins for the custom character go I'm perfectly cool with it.