The technology that builds the web has been continuing to advance but as it does the browsers that we use to experience the web have to be updated as well. With that in mind it's always a good idea to update your browser to the latest version available so that it properly displays web pages for you.
According to studies, there are many people who don't upgrade their browsers to the latest versions and this causes issues for web designers. Designers then either have to create "hacks" to satisfy old browsers or they can't always use the latest technology in making web pages for fear that it will "break" on an old browser.
The most used old browser is Internet Explorer 6. Designers have been waiting several years for it to go away. Now many web citizens are making a stand to kill IE 6 according to a CNN article. Even Microsoft, the creator of IE 6, has been encouraging people to upgrade to their latest browser, version 8. As a designer I'm glad to see the momentum growing to kill IE 6. I think it will free up designers to continue to push the envelope with cutting edge technology and design resulting in an ever improving internet experience.