I Don't Get "Professional Writing"

So, I don't mean professional writing in the sense of like authors who write books, or journalists who write news columns or anything in the sense that your writing is what makes you money. What I do mean, though, is the whole idea of how to write a professional looking document with all the fancy language and the non-use of personal identifiers, and how it has to "sound professional". Basically, like all those papers we had to write for school. This comes in light of my studying for the GMATs and doing a review on the essay section.

Maybe there's a certain merit to looking all polished and such, but given the context of the GMAT the purpose is primarily for communication anyways. Sure, I'd be less inclined to utilize slang, and such terminology, but I'm a little baffled by all this pretense. Why can't I write it like I write this blog? I probably communicate just as if not more effectively this way, and if people were to come talk to me about whatever it is I'm writing about, then I'd probably explain things the way I'm explaining them now. I don't get it.

That's my gripe for the day, I'm done. It's just how things work, and I suppose I have to deal.