Google and other Search Engines assign a value to any given page on your site based on the links, both internal and external, pointing to that page.
(Note that Google PageRank is different from Alexa Rank, and, in my opinion, is much more meaningful.)
The folks at Google firmly believe in Democracy and the Wisdom of Crowds: basically each page indexed in Google (according to the World Wide Web Size website, as of early 2020 there are about 65 billion indexed Web pages out there) is given about 15 votes, and each page splits those votes between its links to other pages. In other words, if a page is just linking back to the home page of its site, it’s giving 15 votes to that home page, but if it’s linking to five other web pages, it’s giving 3 votes to each of those pages. I further believe that PageRank is passed to other pages only when a visitor clicks on a link, much in the same way that the words embedded in a link are associated with the destination page (in other words, if there are five outgoing links from a web page, but users only ever click on the link to the home page of the same website, then that home page gets all 15 of the PageRank votes).
While PageRank isn’t everything, it is a factor, has been copied (albeit imperfectly) by other Search Engines, and is how Google totally revolutionized Search in the late Nineties (up to that point, Yahoo! and other Search Engines just categorized sites with relatively little thought as to those sites’ inherent value).
PageRank is one reason you, as a serious website owner, will want to always be looking for ways to create new content and especially new content spread appropriately over many, many pages. The difference in PageRank between zero and one is just that, the difference between zero and two or three is even better, and thus building a website with 15, 225 or 3,375 pages with lots of internal links between pages is a straightforward way well within your control to create a site with several pages that do at least modestly well within the Search Engine Results Pages, even before accounting for the fact that all that many more pages makes for more targets for traffic, homes for more keyword-rich internal links to other pages within the same site, etc.
Note that these days there’s no accurate way (unless you’re a Google employee) to measure the PageRank of any given page, but the Alexa Toolbar is a decent way of getting a rough idea of the importance of any given website you’re browsing, along with other useful data to use for comparisons).