Greetings?

Why do we greet one another 'Good morning', 'Good evening' and so on?  Is it merely a statement or a mere expression?  If it is mere information, then one need not since the other person too is not such a dumb idiot to notice the goodness of the day.  If merely an expression, then need not be cliched!

I think these bits are a commitment to make the moment good, for myself and for the other too.  It doesn't make any other sense to me!

And I'm still wondering why this thought flashed my mind as sat for tea the other day at table!