- Specifying a licence, author/source and description is mandatory -- the upload just cannot happen if any of them are missing.
- Multiple files can be uploaded at once (up to 50 by default, IIRC) in a fraction of the time you need with the stock upload feature. The more files, the more time you save.
- When uploading several files, you can specify a different licence/author/etc. for each of them if necessary.
- Extensive configuration options.
- More user-friendly.
(1) After installing UploadWizard the vanilla upload feature is still available as Special:Upload in the wiki for those who prefer it. The "Upload file" link in the sidebar, however, is usually configured to point to UploadWizard.