- Has much of your work already done. Don’t reinvent the wheel. Don’t waste time in doing something it has already been done and tested and re-tested by thousands of users.
- Makes you use less code. With less code you get a smaller file size, better maintenance and less development time.
- Makes your code more readable. Which also aids in better maintenance and less development time.
- Makes your web application execute faster. These frameworks are really fast in performing their tasks and most of them have been optimized for really fast execution.
There’s a much better list at wikipedia, if you want to check it out. I didn’t list them here in any particular order, although I would say that I personally like jQuery and ExtJS.
If you’re still trying to decide which framework to use, there’s a comprehensive overview of the most common ones at Ajaxline. You should carefully read that article, and try some of them to see which ones adapt better to your needs.