title: What is the significance of using -U (force update of dependencies) attribute in maven practically?
body: <p>Observation:
  I was testing how Maven retrieves artifacts when building artifacts using <code>mvn package</code> (or) <code>mvn install</code> command. I noticed that Maven was connecting to the Internet to get the dependencies every single time. There was not much time gap (maybe around 1 or 2 minutes) between successive runs as well.</p>
  
  <p>Question:
  Therefore, if Maven connects to Internet (or the repository provided in my <code>settings.xml</code>) every time by default, why do they have a separate attribute that can force update (The <code>-U</code> attribute which is used like this: <code>mvn install -U</code> of a snapshot from the repo? What is the factor that governs/decides between connecting to an external repo or using local repo for retrieving dependencies?</p>
  
  <p>Reference:
  (Please refer <a href="http://www.sonatype.com/node/158?file=books/maven-definitive-guide.pdf" rel="nofollow">http://www.sonatype.com/node/158?file=books/maven-definitive-guide.pdf</a> Page 93 to know more about this attribute). The reference just explains about the attribute but the governing principle behind it is not explained. </p>