The main purpose of capacity planning in IT administration is to ensure that an organization has the necessary resources to meet current and future demands for IT services. This involves analyzing usage patterns, forecasting future needs, and optimizing resource allocation to prevent performance issues. Effective capacity planning helps avoid scenarios like system overloads or outages, enhancing the overall reliability of IT services. By aligning capacity with business goals, organizations can better manage costs and maintain a high level of service for both users and clients.