title: TensorFlow: Understanding the `collections` argument in tf.summary.scalar
body: <p>I am working with TensorBoard, specifically <code>tf.summary.scalar</code>. In the <a href="https://github.com/tensorflow/tensorflow/blob/master/tensorflow/g3doc/api_docs/python/summary.md#tfsummaryscalarname-tensor-collectionsnone-scalar" rel="nofollow noreferrer">documentation</a> it has an arugment <code>collections=None</code>, which is described as:</p>
  
  <blockquote>
    <p><code>collections</code>: Optional list of graph collections keys. The new summary op is added to these collections. Defaults to <code>[GraphKeys.SUMMARIES]</code>.</p>
  </blockquote>
  
  <p>I don't understand this description, and what <code>collections</code> is used for. Can someone please explain this to me, and perhaps point me towards a good example use-case?</p>