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 im

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.