You (the sender) would like someone (the responder) to do you a favor, support some decision you propose or give you some resource you value.  You email the responder, asking him for help.  There is no reply.  Maybe he has an overactive Junk Mail filter or missed the email.  You email the responder again. No reply.  The first time round, you can tell yourself that maybe the responder just missed your request.  The second time, you realize the responder will not help you.  Saying Nothing is the same as saying “No”.

Why not just say No to begin with?  Initially, the responder hopes you do not send the second email.  Then, when the responder reverses roles and asks you for help, you will not hold an explicit No against him.  By the time the second email is sent and received, it is too late – at this point whether you respond or not, there is a “No” on the table and your relationship has taken a hit.  The sender will eventually learn that often no response means “No”.  Sending a second email, while clearing up the possibility the first non-response was an error, may lead to a worsening of the relationship between the two players.  So, the sender will weigh the consequences of the second email carefully and perhaps self-censor and never send it.

Then, Saying Nothing will certainly be better than Saying No for the responder and a communication norm is born.