The main purpose of a cache in computing is to store frequently accessed data temporarily, improving system performance and efficiency. By providing faster access to this data, a cache reduces latency and speeds up data retrieval compared to accessing it from primary storage, such as a hard drive. Caches are commonly used in various levels, including CPU caches for quick data processing and web caches for faster loading times of websites. Overall, effective caching strategies can significantly enhance user experience and device performance.