The purpose of the ALT attribute in the <IMG> tag is to offer a text alternative in case the image is not viewable. There are many possible reasons this could be important:

• The image doesn't load, or is loading very slow


• browser doesn't display the image properly (i.e. inadequate color resolution, lack of support for the image format, etc.)


• Search engines can't "see" images the way humans can, but search engines can read/parse the ALT attribute text to help improve your search status rankings.
  
• The doesn't understand english and is using translating software to read your logo or navigational image(s)


• The user is blind or has impared eyesight and the browser "reads the page aloud" to them


• Any other concievable reason the user can't "see" the image

Even when the image does load, the ALT attribute's text is displayed in a "tooltip" visable when the user points to the image with their mousepointer.

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