Media heavy Drupal sites feel the pain of bandwidth costs in a big way. Simply dropping a few large rotator images on your site's home page, for example, can bloat load times and inflate costs for network and CDN usage on high-traffic sites. Drupal 6's ImageCache module (now built into Drupal 7's Image module) can scale images down to the correct size automatically, but JPEG and PNG images still have a lot of space left to give: specialized utilities can crunch them down to save additional space without compromising quality. That's where the ImageAPI Optimize module comes in: it allows you to pipe all uploaded images through those optimizers automatically.
Installing ImageAPI Optimize is easy on the Drupal side -- drop the module in place, turn it on, and head to the Image Settings configuration page. Normally, Drupal provides a simple 'quality selector' on that page to control the level of compression for scaled JPEGs. With this module installed, however, site builders can specify the specific compression and optimization utilities that should be used to crunch image files as much as possible.
Most of the supported utilities will need to be installed on your web server before they can be used, but if you don't have server access or just want to try things out, you can point the module to Yahoo's hosted Smush.it service. For designers who hand-tweak every image in Photoshop, or make manual use of "Export to Web" compression features in their image editors, the utilities supported by ImageAPI Optimize may not make a huge difference. But when users or non-designers are allowed to post images as part of Drupal content, ImageAPI Optimize can help ensure that you're saving all the space (and bandwidth) you can.