Job requisitions can't be filled in a timely fashion. When requisitions go unfilled for excessive periods of time it means that work deemed important is not getting done. Workloads are sporadic or seasonal. There are jobs that are not full-time by definition. Rather than shuffle and disrupt a work force to accomplish tasks that are sporadic, hiring an independent worker could be a better option.
Growth occurs in unpredictable spurts.
Unlike the regular ebb and flow of work that can be planned, these are situations, like greater than expected success for a new product, where work comes in sudden spurts. Fresh thinking, new ideas are needed. It is human nature to become accustomed to familiar surroundings and situations, and to overlook opportunities for improvement. Objective outsiders can see problems and find solutions that would not occur to workers settled in their routines. In the vast pool of independent professionals there are people who have worked for and been trained by companies that have solved the problems you are wrestling with.