First becoming popular almost 2 centuries ago northern Virginia front porches gradually became almost necessity for medium and large houses to not seem bare. This was before air conditioning, when cooling off outdoors was a frequent necessity. It assisted people to make the daily transitions between their outdoor public world and inner private space. As it became popular, it became a symbol of early Americana.
Andrew Jackson Downing, who popularized the front porch in the US, wrote:
A porch strengthens or conveys expression of purpose, because, instead of leaving the entrance door bare, as in manufactories and buildings of an inferior description, it servse both as a note of preparation, and an effectual shelter and protection to the entrance. Besides this, it gives a dignity and importance to that entrance, pointing it out to the stranger as the place of approach. A fine country house, without a porch or covered shelter to the doorway of some description, is therefore, as incomplete to the correct eye, as a well-printed book without the title page, leaving the stranger to plunge at once in media res, without the friendly preparation of a single word of introduction. Porches are susceptible of every variety of form and decoration, from the embattled and buttressed portal of the Gothic castle, to the latticed arbor-porch of the cottage, around which the festoons of luxuriant climbing plants cluster, giving an effect not less beautiful than the richly carved capitals of the classic portico.
(page 304, A treatise…)
Here’s some more thoughts on the history of the porch in the US in the following NPR broadcast: