Why it was created? I don't think anyone could offer more than guess. Why it is maintained? lol. First, we could get a lot more specific about the kinds of classification and the distinguishing characteristics... but I think we can get away with this: broadly speaking there are two kinds - tactical and strategic. Tactical is almost always SECRET (sometimes with a further qualifier like NOFOR) and below. This is stuff that is very time sensitive. For example: Manning's SIPRNET data dump. It makes sense to protect this information for a short period of time, no more than 2 years - as declassification would only skyline bad actors. Strategic (everything above SECRET) is where are all the highly illegal stuff that would cause a massive amount of backlash is hidden. For example: all of the Snowden/Greenwald releases. Sure, there are some long running strategic programs that could be argued needs protection... but to a degree that justifies the entire security apparatus? No, that is the whole reason why nukes are suited to their purpose - the barrier to entry is far higher than getting your hands on some schematics.

Clandestine sources always pale in comparison to open source collection, when the objective is the truth - not some concealed agenda. So, the idea that the status quo is maintained for any other reason than concealing bad behavior is laughable - we know that it doesn't do what it purports to do.