It's important for all your SharePoint Sites to be as consistent as possible. This helps users' navigation through new pages as they know exactly where to look.
Following these simple rules makes this really easy:
❌ Figure: Bad example - The page on the left has totally different navigation to the page on the right
✅ Figure: Good example - Both pages looking consistent - common navigation elements in the same spot
❌ Figure: Bad example - There are no icons to help users on the left navigation + the headings are the different
✅ Figure: Good example - Icons help users to know what files each link open on the left navigation + the headings are the same