title: What is the purpose of a U-SQL Reducer?
body: <p>I haven't been able to find any documentation or samples for the use of Reducers in U-SQL.</p>
  
  <p>How is a Reducer different from an Applier, because from the function signatures, they both receive one row at a time.</p>
  
  <p>My use case is in the following question:
  <a href="https://stackoverflow.com/questions/35624870/azure-data-lake-analytics-combine-overlapping-time-duration-using-u-sql/35627288#35627288">Azure Data Lake Analytics: Combine overlapping time duration using U-SQL</a></p>
  
  <p>I have achieved this functionality with an Applier.</p>
  
  <p>How can a reducer be more useful for this use case?</p>