Why we use <header> , <section>, <footer> ?

From the point of view how your Webpage looks like, yes, you won't notice the difference of using footer or div tags, but it is very important for search machines when your page is in Internet. Semantic site with such parts used according to the rules looks "better" for them and such one will be higher in search than others. It is good way to do everything right from very beginning if you think about your page's future. And also don't you think it is more clear when you work on your code and maintain it in future (you or any other developer and may be when you come back after a very-very long time) to understand what part is footer or header)) and absolutely different situation if you have not that ones, but a huge amount of similar divs. So it is in very short for SEO friendly semantic sites and nice structure for your HTML. And you mustn't insert header tags and so on if you have no such content on your page. Rules and real project's features have to be balanced together as I think.