If two people are walking along the pavement, heading straight towards one another, and only look up at the last moment, you do not want the pair of them to carry on regardless and walk straight into each other.
On the other hand, you do not want them both to hesitate, step to the same side of the pavement, then back again in some clumsy dance that annoys the hell out of both of them, just for the sake of being bloody polite.
