If it’s simply because the image hasn’t loaded the text often displays in a box in place of the image. If the person is visually impaired their screen reader can read the text out loud or display it in their Braille device.

In the old days web browsers would often display the alt text as a “tool tip” or little message that would appear on top of the image if you left your mouse over it, but this functionality was changed to the “title” attribute since the alt text is primarily for accessibility purposes and should be descriptive, not pointing out some feature in the image or making jokes.