Selecting a document, difference between "choose" and "upload"

When uploading a file there’s a “Choose” button and an “upload” button. The latter is inactive and “Choose” seems to upload the document with no further intervention. What’s “upload” for?

Related: there’s also an inactive “Cancel” button - should that exist in the main page at all? And in the popup version of the uploader, shouldn’t it always be active and be used to close the popup?

“Choose” button opens a dialog to choose a file for immediate upload. “Upload” becomes active when you’ve dragged a document to the upload area and “Cancel” clears the upload list and/or stops any current uploads that are in process.
This section was designed for multiple file upload, even though it currently supports only one file at a time.
Suggestions for improvement?