I honestly had no idea. From Kevin Drum (quoting a reader):
What’s going on in Iowa is that four or five election cycles ago, Republicans decided that the best way to deal with the precinct caucuses for maximum media impact was to simply hold a straw poll at the start of the caucuses and report that to the press as the result. After that, the hard-core insiders would hang around for the actual precinct caucus — the delegate selection phase. The straw poll is non-binding, but there’s kind of a conspiracy of the press and the state party to report it as the result because it comes in earlier and the results are clearer.