title: What is purpose of DataFrame.transform?
body: <p>I see it's the same like apply and have a point only in the context of groupby. </p>
  
  <p>This example from the docs:</p>
  
  <pre><code>df.transform(lambda x: (x - x.mean()) / x.std())
  </code></pre>
  
  <p>Produces the same like:</p>
  
  <pre><code>df.apply(lambda x: (x - x.mean()) / x.std())
  </code></pre>
  
  <p>What is purpose of transform without groupby?</p>