title: When to use select_at, mutate_at, etc. functions?
body: <p>I have been playing around with the <code>verb_at</code>, <code>verb_if</code>, <code>verb_each</code> functions from <code>dplyr</code>. </p>
  
  <p>The examples that I found show usecases only for <code>verb_if</code> and <code>verb_all</code>. </p>
  
  <p>Could someone give some use cases for the rest of the functions.</p>