We'll make this a quick one, since Tiresias (the blind prophet) is only in this story for a second. Basically, prophets crop up all over literature, and they're usually blind.
The idea is that the loss of eyesight gives these people a "second vision." Whenever you find a blind prophet in a story, he or she is probably going to deliver an important prophecy. That prophecy will then be ignored by the characters who asked for it. And ignoring this prophecy will then end in disaster. (Seriously, this is always how it goes down.)
Because of his role, the blind prophet becomes a symbol of fate and the fact that we can't escape it. His ability to tell the future proves that there is a future already in place. When characters ignore the prophet, it's symbolic of a human desire to be in charge of our own fate. Not happening, humans. In Greek mythology, Fate doesn't care what we think.
And for your enjoyment, here are some examples of modern characters that lose their eyesight but gain prophetic vision:
Can you think of any more?