When the wizards of the Web first got together to determine how standardized HTML would work, they thought it might be nice to give some thought to blind people. So they instituted the ALT tag (it's actually an attribute to a tag, but that's more information than you probably need). In brief, the ALT tag contains a snippet of text that appears before the image is fully loaded (in some browsers), or which is read aloud to blind users.
Unfortunately for the blind, ALT tags are used about as often as 8-tracks. They can be handy for promotion purposes, however, so why not kill two birds with one virtual stone?