Because their wasn't much wall space above the door to add a porch at or below the existing eave...
Before adding new Porch
Porch Roof Option #2
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...this owner chose Option #2 from our low eave examples. Starting the new porch roof on the main roof allows the porch eave to maintain the same height as the original house eave.
And what an amazing difference the porch made! The before and after photos are convincing - the new porch truly brings this older home to life. Jenny Lind Spandrels between Turned Posts give this porch a light, airy feeling.
Another example ...
Another example of 1-story with dormers
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This porch roof also starts just below the dormers to maintain adequate headroom at the front eave of the porch.