What is the main purpose of a sandbox in computing?

A sandbox in computing is primarily designed to create a restricted environment where applications can run without affecting the rest of the system. This isolation is crucial for preventing any unwanted changes, whether from malware or poorly designed applications, thus maintaining the overall integrity and stability of the operating system.

By using a sandbox, developers and users can safely test new software or applications without the risk of compromising system security or causing system-wide alterations. This makes it an essential tool for both security purposes and development testing.

The other choices describe functionalities that are generally not associated with a sandbox. Improving system performance relates more to optimization techniques and hardware upgrades. Increasing storage capacity pertains to hardware enhancements such as adding more storage devices or using cloud services. Enhancing user interface design focuses on user experience considerations rather than the confinement of applications. Hence, the correct choice reflects the sandbox's main purpose of limiting the impact of applications on the wider system environment.