As a veteran web developer I have seen huge changes in client requirements and the way we build websites.
In the early days web development was just about creating static documents accessible online, but quickly we needed a way to link these requests into a “session”, and this allowed for user authentication and customisation of user experience.
We built large complex online applications that required user initiated interaction where a request was made to the server and content served to the user. Many developers still use this approach and most websites online today probably fall into this category. To check for changes the user has to refresh.