7 Benefits of jQuery
jQuery is lean and mean. Please forgive the cliché, but it is an apt way to describe the library. The minimized size of version 1.5 of the library is a mere 83KB, only 29KB when gzip'd. That is probably smaller than a single photo on many Web sites, and the browser only has to download and cache it once for use across all your pages. If you use a content delivery network (CDN), the user's browser can cache it across many Web sites rather than download it for each. jQuery packs a lot into a very small size.
jQuery is highly extensible. As mentioned earlier, the core jQuery library is kept tight and focused, keeping out non-essential features. It also provides a plugin framework that makes it easy to extend jQuery, and there are both official jQuery plugins as well as thousands from third parties. This way your page has to download only the specific set of features it needs. Any time you need a feature not included in the core jQuery library, there is a good chance that someone has already written a plugin that does what you need.
TIP: You can find a list of available plugins at jquery.com, the home of jQuery, with user ratings to help you find the best ones.