Why is a user agent string important?
A user agent string can tell a lot about the system environment of the visitor. For instance which brand of browser, which operating system and what browser version the visitor is using. This information then allows the website programmer to make sure the website functions flawlessly for that specific version of the browser.
A user agent parser can be used to redirect a user of a particular browser type to a different page, or to program a software exception that allows website visitors who are using an older browser to see an alternative version of the website.