Jakob Nielsen describes in the first article of the Revelation of the user experience Principle: Let the user always know what is happening now.
Users want to get a response immediately, but there are always cases where the site takes a while to wait for the operation to complete.
Then, the interface should be displayed in the background graphics, reflecting the percentage of completion. Or play the sound, so that users understand what is happening.
This principle is also related to the file transfer: Do not let users feel bored, give them a look at the progress bar. Even if it is not a pleasant notice, such as transmission failure, it should be in a pleasing way to show. Let your users smile!