A "page" exists because paper exists. When you aren't using paper why would you break up your content into arbitrary fixed size increments, rather than using (say) "sections", which are the size of the content you put into them?
Second, because you want to print -- in which case, your artifact ends up in a page.
Third, because a screenful, even if dynamic (based on resolution, etc) is still a kind of a "page" (in HTML parlance, a "viewport"), and you might want to design to take advantage of what fits in one.
I've tried using it for my resume since may about 10 years ago, to make it possible to convert it to PDF with a standard browser, but to this day many of these features aren't fully supported.
That's kinda the main selling point of software like Prince, which has implemented all of this ages ago, but it requires a licence.