It’s perfectly acceptable within its dialect. The study of language isn’t about applying rules anymore. It’s about capturing how a linguistic group speaks today.
meant to replace the above with this, but timed out:
It’s perfectly acceptable–grammatical-- to the speakers of its dialect. The study of language isn’t about applying rules. That was school days. Academic study of it is about capturing how a linguistic group speaks today. You and I aren’t saying anything different per se, of course. But in a prescriptive mode, you are adopting the standard dialect–and beyond, with specific formality. In Linguistics, no dialect is the one that’s correct
I know a great career for you–teacher.
Oh, I agree, but I feel that this particular instance–to the way my brain parses it–fully fits in with the traditional prepositional analysis. You hear it differently. That’s fine–it’s not an exact science.
I’m asking what the purpose is. It “serves the same purpose of ‘at’ in 'where are you at?”
It is a real question. What purpose does it serve?
Several posters in this thread, including myself, have already pointed out, in one way or another, that they serve as discourse particles.
You may not realize it, but a large part of language isn’t representational.
I own a vintage copy of “Wings Complete” published by Hal Leonard,which I purchased new in 1977. The published lyrics clearly say “in which we’re living”.
Not sure why Big T’s comment is quoted along with mine, but I wasn’t speaking of purpose at all.
I will now though. 
Not every element in an utterance has an explicit or exclusive purpose. Natural language strives for redundancy as well as efficiency. Both serve to maximize communication/understanding. The ‘at’ and ‘to’ we’re talking about here seem to me to be added redundancy. Another example of redundancy in English is how the subject and verb of a sentence are both marked as singular or plural: A dog barks. Dogs bark. Note that even the article adds redundancy.
Yes, I agree that these particles (to, at) serve for communication, but I would not say simply as redundancy for redundancy’s sake.
I’ll repeat what I mentioned above: Much of language usage is not strictly representational. The discourse particle at in Where are you at? is not simply a repetition of the locative semantics already conveyed by where. As Wikipedia describes it,
[QUOTE=Wikipedia]
[A] discourse particle is a lexeme (word or phrase) or particle that adds no direct semantic meaning in the context of a sentence, having rather a pragmatic function: it indicates the speaker’s attitude, or helps structure their interactions with other participants in a conversation.
[/quote]
Maybe a more illustrative example for this thread is the case of downtown. Go collocates with downtown in a similar way as it does with where: it doesn’t require to. So we can imagine this likely exchange between two bus riders:
A: I thought you said this bus is going downtown!
B: No, I didn’t say it’s going TO downtown; I said it’s coming FROM downtown.
Speaker B’s use of to here is only “redundant” semantically. Pragmatically it serves a separate and distinct purpose. And there are other circumstances where speakers will use these particles for much more subtle reasons. Most linguists agree that if there are two acceptable but distinct ways to say something within a discourse community, there’s usually a reason for it. (I.e., it’s not simply redundancy.)
I’ll say it again: Much of language usage isn’t representational. The past tense isn’t always about the past. Deixis is not always about proximity. Diminutives are not always about size. There’s a lot more going on with how we use language; it’s a lot more complex. If we say these particles are redundant just for the sake of it, we’re kind of ignoring a big part of the story.
Even with the article mentioned in the example above about dogs and barking, I don’t think the singular marking is purely redundant, when you examine how it’s used in context – it isn’t there just to indicate a distinction between singular and plural. To throw out an example along the same theme: *A dog from a house near mine was barking all last night. I don’t know why, but the dog just wouldn’t shut up. Whatever the reason, its owner should know what’s happening and do something. Honestly, by 3:00am I was ready to kill that dog, if given the chance. I understand dogs will naturally just bark from time to time, but when a dog goes on like this for hours, there’s something wrong. Clearly this dog has some kind of problem.
*The articles range and serve various discursive or affective purposes, which show that, pragmatically, articles do much more than simply indicate number (singular or plural).
a dog – first mention
the dog – shared knowledge
that dog – negative affect
Ø dogs – universal plural
a dog – universal singular (not first mention, as in the beginning)
this dog – positive affect.
I don’t think the verb conjugation (or the noun) in a dog barks makes the singular marking of the article “redundant” – it allows a distinction we don’t get, for example, between the dog and the dogs, and it’s why some dog can carry a distinct connotation. By the way, this is the only conjugation left in all of the English language. Even today, in language instruction, there’s way too much obsession with verb forms, when these other things which also constitute true proficiency in language get ignored.