When the web was created, they consist of pages, that was it. Whoever created the page put content on the page(s), that was a website.
when you had to navigate to a different page, a whole new page page had to fetched, loaded and displayed.
slowly, this process became more dynamic. Databases where introduced (allowing for user content on a website), but still the entire page had to be refreshed to then fetch all post and display them.
Take this forum for example, the “old way” meant that after submitting my reply, the whole page needed to be refreshed to display all replies
Too speed up this process, wouldn’t it be much nicer if my reply was submitted, validated and then just appended to the page without a page refresh?
reactJS (and others) takes thing even further, we named it SPA (single page application). Its so dynamic, you only update what you need, there are never any page refreshes.
So render only need to be called a single time, from that point on-wards you can do everything dynamic with react.