The alt field specifies the text to be shown in the container when the resource is missing. If you don't use it you'll get an eyesore in the form of a broken image icon, within a reserved space for the resource. The alt="" instructs the browser to minimize space reserved for a missing resource. This is indeed useful for layout elements that are only a few pixels on each side. This way you don't mess up layout. It's used to keep the UI usefull when things go missing.