Compared to the previous methods we used to add CSS, the advantage to using this external stylesheet method is flexibility.
We may have multiple HTML files linked to the stylesheet, but as long as we keep our CSS in a separate file, any styling changes or additions we need to make are done in one file.
Another advantage is that the browser often caches the stylesheet after the first request, which saves on download time throughout the site for repeat visits.
Are you wondering why we write CSS or link to stylesheets from the head section of the HTML document? This is so we load the CSS before the HTML markup.
Also, it's standard practice to link to stylesheets from the head of the document. That way, the browser loads and parses the stylesheet first, before any of the body elements.
This means that when our website is up on a web server and a user visits our site, the HTML is nicely styled right before it's presented to the user.