A great optimisation for many modern websites is to farm out the hosting of linked items, such that the client browser can receive your base page code (HTML) and then pull media in from a number of directions.
Adding a library this way:
- Reduces the load on your server- one less thing to push.
- May well be quicker for the client- Google have a fast worldwide architecture, and the library may also be cached from a visit to another site.
- Means less code for you to manage (and FTP up).
On the downside:
- You’re reliant on an additional system- which adds an extra potential failure point to the process (this may make a page more likely to part-load than completely fail – which may or may not be advantageous).
Google currently host multiple versions of JQuery, JQueryUI,Prototype, Script.aculo.us, MooTools, Dojo, SWFObject, YUI library, Ext Core, and their own native APIs.