When it comes to intro paragraphs, a lot of the examples have broad "accepted knowledge" statements like the ones here. It's called an attention-getter. The idea is that the substantiation part comes later, when you're trying to prove the less-accepted fact. I know, it's silly. Imo, serious academic writing shouldn't rest on gimmicks, but it's hard to break old habits, and I still find myself doing it on occasion (just wrote a paper on children in The Canterbury Tales, and the intro drove me nuts because I wanted to talk about the old notion of medieval children being mini-adults).