Well the <head> tag includes important information about a webpage, that isn't actually part of the webpage content. Like the title. The title is like the title of a book, you don't put the title in the opening paragraph, you put it on it's cover, or title page. The <body> tag defines where the actual content starts.
The <html> defines where the whole html document starts and ends. There are actually a couple things that can come before the <html> tag. Things the further define what kind of html is up ahead.
So it's required for the browser to understand what's going on, to store extra information about the webpage, that isn't part of the content, and well because, it's a Markup language and that's what a mark up language does. It defines text by sections.
