By 1777, Philadelphia was America's most important urban hub. A bustling port of about 35,000, the city was also America's nascent manufacturing center. By 1775, the foundries of Philadelphia were already casting cannon of iron and bronze. According to Boatner, "4,000 stands of arms" were also manufactured in Pennsylvania by the winter of 1775. Local forges cast the iron products needed in martial times: cannonballs, swords and bayonets, entrenching tools such as shovels, and horseshoes.
Naturally, gunpowder was needed to fire cannon and bayonets. Five powder mills in the region provided the bulk of ammunition that Washington was now using.
Bass Otis after Thomas Thompson, c.1858, Portrait Gallery (Second Bank) Thomas Paine
Philadelphia was also home to the first paper mill in America. And paper proved surprisingly important. Paper was used to make currency and paper cartridges, the packets which held gunpowder. Paper, of course, was also needed to make books. And books such as Thomas Paine's "Common Sense" had an immeasurable positive impact on the Patriots.
Textiles, most importantly canvas for sails and tents, were sewn in Philadelphia. Blankets, too, whether made in factories or in homes by Patriotic women came from the city.
At the time of the Revolution, Philadelphia was also the most medically advanced city on the continent. Thus the medicine and medical supplies made in Philadelphia helped the war effort.
Further, the long-established and productive farms to the south and west of the city provided the Continental army with food, flour, horses, and livestock.
Finally Philadelphia was the seat of the Revolutionary Congress. If Howe could drive the government from the city, he could disrupt the flow of communication between Congress and the army. Requests for supplies, money, arms, etc. would have to be re-established wherever Congress decided to settle.
Howe must have felt that taking Philadelphia would deal a crippling demoralizing blow to the Americans.