Things like armor and bullet proof do reduce blast damage, but hitting something with blast damage does get around a couple other defensive abilities. First of all you aren't making a Df duel to do some damage; if there are two models close together and one has really high defense, often it's better to aim for the lower defense one and blast over to the higher Df model. Also, can get around things like Terror or Manipulative through the same means. Another big one is that since there is no duel on it, the blasted model won't get any defensive triggers, so it's a great way to get around annoying defensive triggers (such as Molly, Dreamer, Marcus, etc.).