Sheep are not known to be overly bright, and therefore, are vulnerable to many perils. The shepherd's life was hard, as he had to care for the sheep, including finding them food and water, brave the elements, and defend them. Sheep were also subject to sickness, injury, theft or loss. Your sheep and goats have not miscarried, nor have I eaten rams from your flocks. And you demanded payment from me for whatever was stolen by day or night.
Sheep are powerless to help themselves and have to be defended.
The staff is used as a metaphor in the Bible as a symbol of both defense and comfort. For protection at night, sheep were kept in a sheepfold, an area surrounded by a stone wall and topped with briers to keep predators out. Often the sheepfold had a protected area included, such as a cave, for protection from the elements. The gate was guarded by the shepherd, where he slept at night to keep the sheep in and others out. Even today, if the sheep are prone to stray, the shepherd ties one leg of the animal to this tail.