The main function of the universal selector in CSS is to select all elements at once, allowing you to apply the same style to all elements on a website. It is useful for setting margins, paddings, or overall font styles uniformly. However, caution should be exercised when using the universal selector as it can affect all elements and potentially slow down website performance. It is commonly used for setting general style rules or temporary debugging. The use of the universal selector continues in the CSS3 version expected in 2023