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?