This is one of the core principles of web design.
when you enter a new building for the first time, we try to figure out the layout of the new building often unconsciously. And once we know where the exits, elevators, hallways, etc are we feel at ease. When we go up a floor we expect the layout to be the same.
This is the same with websites. When people enter your site, the first thing they do is to figure out the layout of the site. Where the navagation menu is, what the major sections are, etc. As they navigate the site, they expect the layout to remain the same. If the layout changes from page to page, the get confused.
So it is important to have consistent layout across all pages. This can be accomplished by first creating a dummy page with all core elements and then creating individual pages from the dummy.