They are convenient. Common website layouts have become popular because users have proven that they can use them. you know very well
A great user experience comes from building rapport with your users. Visitors experience a pleasing sense of deja vu when they see familiar features laid out as expected. As a result, people are spending their time digesting the content instead of focusing on the unique design of the page.
You can save money, reusing existing web layouts saves time. Designers can spend less time trialing and erroring layouts and can focus on visual hierarchy and other aspects of design that directly affect the user experience.