The purpose of a software development lifecycle (SDLC) is to provide a structured approach to designing, developing, and maintaining software systems. It outlines the stages through which software progresses, including requirements gathering, design, development, testing, and deployment. Each phase ensures that the project is well-organized and meets quality standards. By following an SDLC, teams can manage risks, reduce costs, and deliver functional software on time, enhancing collaboration and communication throughout the development process.