1) It establishes to both parties that you have a sense of self - worth and confidence. If the other person really cared about you, they would have contacted you by now. Since they have not, it shows that they aren't dealing with the situation in a mature, adult manner, or that they really didn't care about you as much as you thought. Either way, it's time to say goodbye and the way you do that is to *cut off all contact*.
2) Often, people who are involved in online relationships that fail attempt to relegate the relationship to the "let's just be friends" level. They will follow you around like a puppy, making you emotionally miserable because they still think "you are a friend". They simply won't "get the message". Make no mistake - if you've been romantically involved with someone and it blows up like this, you've been FRIENDZONED. Blocking in cases like this is an act of kindness - both to yourself and to them.
3) It's just good business practice. When the offending party has been blocked, you don't have to worry about them reading your content and you are free to move on with an open heart. It may hurt for a while, but it will go away.
Been jilted by someone? Try my solution. You'll thank me later!