Upload Wizard uses chunked uploads to provide better reliability during the upload of large files. In addition, it allows a greater maximum file size (4 GB as opposed to the usual 100 MB).
During a chunked upload, the file is split into small parts and these parts are uploaded. If some parts failed to upload, the whole file does not have to be re-uploaded, only the parts that failed. All this is done in the background in UploadWizard.
It provides better reliability not just for UploadWizard, but also for bots using the API, especially for files uploaded through the API.