Developers use virtual machines to create isolated environments that mimic different operating systems and configurations, allowing for consistent testing conditions without affecting the actual hardware or primary operating system. This isolation is key for managing dependencies, avoiding conflicts, and ensuring tests run in a controlled environment. Running legacy applications isn't the primary goal of test development, nor is saving hardware resources—a virtualized environment often requires significant resources. Accessing multiple operating systems simultaneously, while a benefit of virtualization, isn't specific to the test development process.