The Render Instance allows to change parameters of the Composition just for this render instance. For example, you may change the composition Format or framerate. Additionally, as seen on the screenshot above, any custom parameter created on the composition can be overriden for this render instance.
Additionally, you can specify an Audio File that will be encoded in the video. Note that this is a temporary feature as Autograph will soon integrate Audio directly in the Composition. Note that you can also control whether you want to output Progressive or Interlaced video.