When images are uploaded from the Image Gallery content type, which optionally allows uploads from members, the images are optimized in order to prevent members from taking up all of your file storage space very quickly and to reduce the time it takes people visiting your site to download them.  Some browsers won't even begin to show the rest of your page until all of the images are downloaded.
Especially in the case of screen shots that games output, or photographs that are taken with high-quality cameras, this can result in great savings of space with very little noticeable reduction in quality.  However, if the images are already highly optimized from the source, or from some other image editing software, the quality may be noticeably impacted.
Those with Super Admin access to the site who do not want an image optimized on upload should upload them directly in the Control Panel's File Manager, where they have the option to optimize (or not) upon upload.