We can look at Cascading Style Sheets (CSS) from a number of contextual perspectives. I prefer to view them as a correction to a fundamental mistake that was made at the beginning of Web Time, back in the old days of the mid-1990's when Tim Berners-Lee and a subsequent phalanx of Web builders first envisioned the beginnings of the Web.
What was that mistake?
Almost as soon as the Web became popular, graphic designers began noticing what they saw as a fundamental flaw: the method by which a Web browser displayed information in HTML files was not within the designers' control. No, it was the users who were in primary charge of how the Web pages they visited would appear on their systems.
You can see that as you keep adding content to this page, it adds nicely boxed and centered material down the center of the page.