Because less time needs to be spent on generating the HTML, the page will show up faster on visitors' devices — also mobile devices.
However, you of course still want pages to be personalized. Different visitors have different permissions (and hence different contextual links), different interests (and hence have "new" or "updated" indicators in different places), and so on. In Drupal 8, we've introduced the necessary foundations to easily serve the same HTML (well, >90% the same — i.e. the most expensive parts), yet still personalize the page by using client-side caching (
sessionStorage) in combination with clever client-side cache invalidation.
The end result: really fast websites, with superb perceived performance!
Note: where possible, I've explained how to achieve the same in Drupal 7.