Many WordPress themes, especially those with “magazine-like” layouts, use an image to represent each post. It might just be on the front page. It might be alone, or alongside an excerpt. Until now, there was no standardized way to do this. Many themes would require you to tediously enter a Custom Field with the value being the URL you wanted to use. Often you had to do cropping yourself. With WordPress 2.9, theme authors can easily enable Post Thumbnail selection UI and call those image using simple template tags.
Whether you’re developing WordPress plugins, WordPress themes, or rolling out a new site based on a customized version of WordPress, it’s often helpful to be able to mirror your live site on your local system for development. This is common for commercial sites, where you have a live production server but you also need a development and/or staging server, which is run locally to test plugins, mods, themes, and everything else that you don’t want to do on the live production server. For example, your live site could be http://www.example.com/blog and your development site might be http://localhost/example.com/blog. However, WordPress serves pages with embedded absolute URLs based on the absolute site URL configured in your database, so none of the links on your development site will work.