Alternate text for images.

    Why is it necessary to provide alternate text for all images?  I
have been using JAWS for windows with Internet explorer.  For example,
at the top of every page on the kingston regional police site there is a
banner surrounded by two thin blue lines.  I feel that the lines are
mostly aesthetic.  Having 'thin blue line' read to me every time I visit
a page doesn't mean much to me, it just takes up time.  Is there a
situation where alternate text for those thin blue lines would be
useful?

Thanks,

    Andrew.