During the 18th century, military tactics primarily included a respite from fighting during the winter months mainly because of the severity of the weather. The Army was not sure of where they were heading when they moved into winter quarters. Valley Forge was chosen for many reasons. Here are a few:
The British were occupying Philadelphia for the winter. The American Army needed to keep an eye on them and prevent them from foraging in the countryside for food they needed.
The Congress, after fleeing Philadelphia, were at York PA, west of Valley Forge. The troops being at Valley Forge between the British and York were a protection for Congress
The Valley Forge area (terrain) was defensible. The grounds were suitable to guard against attack. While the conditions were harsh, Washington tried to provide for the comfort of his men. The lack of supplies, clothing and food meant the winter here was not a pleasant one!