A CloudSim developer designs, builds, and maintains applications using the CloudSim software framework, a tool that simulates cloud computing infrastructure and services. They create models of data centers, virtual machines, and resource provisioning policies to analyze and understand the performance of cloud-based applications in a controlled environment. Through these simulations, they can test scalability, reliability, and efficiency of cloud services without the cost of deploying them in real-time. They also use CloudSim to research and develop new cloud computing technologies, strategies, and policies. In essence, they innovate and improve cloud solutions through simulation.