I would like to answer the question mentioned in the subject line in simple words.

SEC is the regulator of the US stock market. Without the absence of SEC, anyone would register a company  and list it to stock market to raise funds without having a real profitable business. That brings a significant risk because without a regulator anyone would raise money and run away without being caught.

So SEC is here to ensure that only real and profitable companies are listed in stock market so that the chance of being cheated are minimized. That's the real world usage of SEC in simple words.