It's common for people to write the way they speak. If you proofread you may find times you do it yourself.
Heck, a Biblical instructor told us the hideous repetition in, for example, the Book of Ezekiel is likely do to the scribes doing just that.
If you've never read it, do so. It's actually worth the laugh. No disrespect intended. It's just UNBELIEVABLY REPETITIVE TO THE POINT OF BEING SENSELESS.
It is possible to begin a sentence with the word so, and it is used as a conjunctive adverb. It modifies the entire sentence that follows it, and in some cases is synonymous with therefore or ergo. In most cases though, I think it is probably used as a discourse marker. I personally find it more of a filler than anything else.
A reasonable command of the English language is neither something to feel conceited or elitist about. I have no problem with speedy, casual and colloquial communication that sometimes forgoes grammatical details such as the omission or misappropriation of punctuation and the occasional spelling error.
If the communication is egregiously careless, ignorant or unintelligible only then may I begin to feel ire for no other reason than the inefficiency and ambiguity it introduces. That doesn't make them appear any more or less credible to me, their message determines that.
Also notice how self-conscious this thread has made everyone, so much as to put them on their best behavior. Casual, personable and natural is better IMO.
Or even worse, "OK so".
Why do people do this? I understand those things being used as superfluous fillers or preambles in speech, but why is it necessary to use them when writing?
OK, so why is the title of your post a sentence fragment? Your first sentence starts with a conjunction. I can sit here all day and point out nitpicky syntax errors, and I would be kicked off the planet if I started in on semantics. We are all writing for an audience when we post online, but I don't let minor errors get in the way of communication.