Often it isn’t cost effective for a small business to spend money doing extensive user testing. Utilizing the testing other companies in that market segment allows easy identification of trends. There is no guarantee that large websites have done user testing, but if they have a large enough audience, they will become the learned convention for certain actions.
In addition, if you are trying to see when a page was written because the date is important but the author/website decided to omit that crucial information, you can use one of these tools to see when the page was first indexed. It might not be the exact date, but will likely help you get in the ballpark.