The HTTP Capture feature creates static requests for any objects generated through a particular plug-in on the page, for example Flash or Silverlight. If a Flash object on the page generates images or objects, HTTP Capture manually calls those objects to be downloaded.
When Enable Flash Playback and Enable Silverlight Playback are selected in the playback settings, HTTP Capture becomes redundant and you may see duplicates of the objects if HTTP Capture is also selected. This is why this feature is set to Off by default. The only time you need to use HTTP Capture is when Flash or Silverlight interaction is not working properly and therefore some objects need to be called manually.