s3d is an XML schema for 3d scenes, objects, and so forth (intended primarily but not exclusively for computer games). The problem is that often as a games developer you need to work with many 3d editing tools and rendering engines, and there should be an easy and portable way to move objects, levels, etc. around. By having a common intermediate file format, you only need to write one exporter per tool and one importer per graphics engine. Also, you can develop generic tools which are not tied to any particular engine.