So is supposed to be used in something like, "The grass is tall, so it will be mowed." The use expanded to "The grass is tall. So, it will be mowed."
Now, so is commonly used at the beginning of a sentence to mean "as a result" as it was traditionally used, but also with the same meaning as "uh," as an initial attention-getter. For example, "So, do you want to go get some lunch?"
It is also used sometimes in a discussion to "hold the floor," or keep one's side of the conversation going by making some noise between sentences. This is particularly common in public interviews.
So is sometimes used in the beginning of a sentence to connect the sentence with the previous sentence or paragraph, as a discourse marker. It may imply that the content of the sentence is there because of the previous idea, or it may just be there to keep up the rhythmic flow of the text.
So, I find it annoying, too.
It's partly a regional usage: Seamus Heaney in the foreword to his translation of Beowulf says
Conventional renderings of hwæt, the first word of the poem, tend towards the archaic literary, with ‘lo’, ‘hark’, ‘behold’, ‘attend’ and – more colloquially – ‘listen’ being some of the solutions offered previously. But in Hiberno-English Scullion-speak, the particle ‘so’ came naturally to the rescue, because in that idiom ‘so’ operates as an expression that obliterates all previous discourse and narrative, and at the same time functions as an exclamation calling for immediate attention. So, ‘so’ it was:
So. The Spear-Danes in days gone by and the kings who ruled them had courage and greatness. We have heard of those princes’ heroic campaigns.
(full text here; http://www.wwnorton.com/college/english/nael/beowulf/introbeowulf.htm
Both responses from - http://english.stackexchange.com/questions/43273/sentences-beginning-with-so