First and foremost, Virginia State Law, or the Code of Virginia, requires that all local governments have an adopted Comprehensive Plan, which spells out policies for future development to ensure orderly growth and the protection of the public health and welfare of the community. The authority for comprehensive planning in Virginia is found in §15.2-2223 of the Code of Virginia, which states, "The comprehensive plan shall be made with the purpose of guiding and accomplishing a coordinated, adjusted, and harmonious development of the territory which will, in accordance with present and probable future needs and resources, best promote the health, safety, morals, order, convenience, prosperity, and general welfare of the inhabitants..."
Secondly, there is great value to a community in developing a Comprehensive Plan and the process itself can be as valuable as the final document. The planning process allows a community to be proactive (vs. reactive) to issues and changes that arise over time, provides guidance for orderly growth and development, provides an opportunity to think regionally, serves as a guide from which to base all decision-making in the present and over time (regardless of turnover in leadership), and can set up the community for grant funding success.
A bit more background, for all the history buffs out there:
Two laws from the 1920s created the legal precedent for comprehensive plans—the Standard Zoning Enabling Act and the Standard City Planning Enabling Act, both offering details about the scope of comprehensive planning powers at the local level. Each state provides further mandates and recommendations about the necessary components of comprehensive plans.