The third introductory paragraph above is from a published paper by me, John Hodgson, and so I feel confident in explaining how it works.
The first sentence starts the paper in quite a striking way. It suggests that something significant happened that was important in relation to the overall topic of the paper. The second sentence explains what this was: that Dr King expressed concern about the effect of television, films and comics on the literacy of the British people. This is both interesting (because we still have such debates today) and, most importantly, it suggests that I've done my research and know what I'm talking about. In other words, it makes me sound like an authority on the subject. This is one of the main purposes of the introduction: to give the reader confidence in the writer. I continued to give grounds for confidence by showing my historical knowledge of the Birmingham Centre for Contemporary Cultural Studies. Then the last sentence of the introductory paragraph indicates my thesis. I will be writing about the way in which Hoggart's book anticipates contemporary debates about literacy, media and cultural policy. My thesis has three parts (debates about literacy, media, and culture) which makes the direction of the essay clear to the reader and will help me to write it in a logical manner, dealing with each of these aspects in turn.