The user agent tells the page what the browser is, so the page can tell whether you're runnit Chrome, Firefox, Safari, etc. The intent is for the page to change behavior depending on the browser since each have different capabilities (web standards change quickly). Unfortunately, pages rarely get updated in a timely fashion when browsers implement web standards so the engine check is frequently inaccurate.

Changing the user agent means changing what web pages think you're running. If a page uses an optimized API on Chrome and a slower one on Firefox because Firefox was slower to implement it, then you can get a speedup by saying your Firefox is Chrome. Some pages refuse to run unless it's a specific browser, so lying can make those pages work.

I hope that makes sense.