Because the structure is the skeleton of all web pages. This structure is a language that allows the computer to read and interpret a raw texte. The structure device that provides to access another text through the links. Without this structure the raw text would be empty; without structure. The structure defines the conent of the text.
You spelled “text” wrong michelmambert. I suggest you study more language arts, as to further increase your knowledge in how to spell different words correctly.
Raw text, at a very simplistic level, is the text you read on a web page or in a document (in say Google docs). Raw text conveys some information and the writer of raw text usually follows the syntax/grammatical rules of the language, say English, if the text is to have some meaning.
Now when the raw text is seen or rendered in the web browser or say Google Docs, you will observe that the text has some structure to make it more organised and easier to read and understand. For example, it will probably have some titles and headings, will be organised into paragraphs, might have sections in italics, some text in bold and so on. The writer of the raw text will use the tools and facilities offered by the text editing program to create such structure. The editor will save this structuring information in some way along with the raw text so that it can be rendered appropriately when the document is displayed.
HTML is one method to store structuring information along with the raw text. Web browsers parse the HTML and render the document with the structured that was imparted to it when it was created.
Like I said - a simplistic explanation!