Everyone wants their website to be accessible to as many people as possible. This means at least supporting four major desktop browsers (Chrome, Internet Explorer, Firefox, and Safari) each with several active versions. Cross browser testing is all about being able to test your website across all major browsers with almost the same effort as testing just one browser.
Traditional testing tools depend on specific hooks within the browser, and code-level details of your website that differ between browsers, meaning that you need to create test automation scripts per-browser (and sometimes per-browser version). Effective cross platform browser testing significantly reduces the time it takes to create and maintain test automation scripts for your website across all major browsers and versions.